Help: Rework get_filename_component docs

I know, I know, it's a deprecated command. But I found myself at the documentation for get_filename_component() and it just struck me as confusing in both layout and content. The .. versionchanged notations were especially unclear.

So, I kind of... totally restructured it, along the lines of list(), file(), and other docs of that sort. I tried to make the different modes both more distinct from each other, but also clearly different forms of the same command (as shown in the opening synopsis section).

I'll be glad to make any changes necessary, I definitely tried a few things that will be very much down to personal taste so not everyone will love them. (Not sure I love them, in all cases. But no harm in experimenting.)

Topic-rename: doc-get_filename_component