[translation] ux study docs feedback

[kristapratico]

• revisit DocumentTranslationInput naming. Maybe DocumentTranslationConfiguration makes more sense?
• fix poller.result() docstring to be more doc translation specific and explain that calling it completes the LRO. (docs won't allow this)
• improve docstring for begin_translation - format such that the two options for input are numbered like 1) 2) to give a better idea that there are two ways to pass inputs
• follow up on why "wl" permissions no longer overwrite blobs in containers