doc(cargo-tree): mention it considering feature unification

src/doc/man/cargo-tree.md

@@ -53,6 +53,23 @@ turn depends on `cfg-if` with "default" features. When using `-e features` it
 can be helpful to use `-i` flag to show how the features flow into a package.
 See the examples below for more detail.
 
+### Feature Unification
+
+This command shows a graph much closer to a feature-unified graph Cargo will
+build, rather than what you list in `Cargo.toml`. For instance, if you specify
+the same dependency in both `[dependencies]` and `[dev-dependencies]` but with
+different features on. This command may merge all features and show a `(*)` on
+one of the dependency to indicate the duplicate.
+
+As a result, for a mostly equivalent overview of what `cargo build` does,
+`cargo tree -e normal,build` is pretty close; for a mostly equivalent overview
+of what `cargo test` does, `cargo tree` is pretty close. However, it doesn't
+guarantee the exact equivalence to what Cargo is going to build, since a
+compilation is complex and depends on lots of different factors.
+
+To learm more about feature unification, check out this
+[dedicated section](../reference/features.html#feature-unification).
+
 ## OPTIONS
 
 ### Tree Options

src/doc/man/generated_txt/cargo-tree.txt

@@ -44,6 +44,25 @@ DESCRIPTION
        it can be helpful to use -i flag to show how the features flow into a
        package. See the examples below for more detail.
 
+   Feature Unification
+       This command shows a graph much closer to a feature-unified graph Cargo
+       will build, rather than what you list in Cargo.toml. For instance, if
+       you specify the same dependency in both [dependencies] and
+       [dev-dependencies] but with different features on. This command may
+       merge all features and show a (*) on one of the dependency to indicate
+       the duplicate.
+
+       As a result, for a mostly equivalent overview of what cargo build does,
+       cargo tree -e normal,build is pretty close; for a mostly equivalent
+       overview of what cargo test does, cargo tree is pretty close. However,
+       it doesn't guarantee the exact equivalence to what Cargo is going to
+       build, since a compilation is complex and depends on lots of different
+       factors.
+
+       To learm more about feature unification, check out this dedicated
+       section
+       <https://doc.rust-lang.org/cargo/reference/features.html#feature-unification>.
+
 OPTIONS
    Tree Options
        -i spec, --invert spec

src/doc/src/commands/cargo-tree.md

@@ -53,6 +53,23 @@ turn depends on `cfg-if` with "default" features. When using `-e features` it
 can be helpful to use `-i` flag to show how the features flow into a package.
 See the examples below for more detail.
 
+### Feature Unification
+
+This command shows a graph much closer to a feature-unified graph Cargo will
+build, rather than what you list in `Cargo.toml`. For instance, if you specify
+the same dependency in both `[dependencies]` and `[dev-dependencies]` but with
+different features on. This command may merge all features and show a `(*)` on
+one of the dependency to indicate the duplicate.
+
+As a result, for a mostly equivalent overview of what `cargo build` does,
+`cargo tree -e normal,build` is pretty close; for a mostly equivalent overview
+of what `cargo test` does, `cargo tree` is pretty close. However, it doesn't
+guarantee the exact equivalence to what Cargo is going to build, since a
+compilation is complex and depends on lots of different factors.
+
+To learm more about feature unification, check out this
+[dedicated section](../reference/features.html#feature-unification).
+
 ## OPTIONS
 
 ### Tree Options

src/etc/man/cargo-tree.1

@@ -52,6 +52,21 @@ In this tree, \fBmyproject\fR depends on \fBlog\fR with the \fBserde\fR feature.
 turn depends on \fBcfg\-if\fR with "default" features. When using \fB\-e features\fR it
 can be helpful to use \fB\-i\fR flag to show how the features flow into a package.
 See the examples below for more detail.
+.SS "Feature Unification"
+This command shows a graph much closer to a feature\-unified graph Cargo will
+build, rather than what you list in \fBCargo.toml\fR\&. For instance, if you specify
+the same dependency in both \fB[dependencies]\fR and \fB[dev\-dependencies]\fR but with
+different features on. This command may merge all features and show a \fB(*)\fR on
+one of the dependency to indicate the duplicate.
+.sp
+As a result, for a mostly equivalent overview of what \fBcargo build\fR does,
+\fBcargo tree \-e normal,build\fR is pretty close; for a mostly equivalent overview
+of what \fBcargo test\fR does, \fBcargo tree\fR is pretty close. However, it doesn't
+guarantee the exact equivalence to what Cargo is going to build, since a
+compilation is complex and depends on lots of different factors.
+.sp
+To learm more about feature unification, check out this
+\fIdedicated section\fR <https://doc.rust\-lang.org/cargo/reference/features.html#feature\-unification>\&.
 .SH "OPTIONS"
 .SS "Tree Options"
 .sp