Reformat unordered lists in all docstrings

WIP for #2411. Pushing this to get initial feedback.

I did a visual check of the rendered docs (generated by stock html formatter and my pandoc+sphinx combo) for some pathological modules (File, GenServer, Module, Regex). They seem to be consistent.

/cc @ericmj @jwarwick

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List items are marked with a star (*). Bullets marking list elements
must have a 2-space indent. The continuation lines should be indented
by 2 more spaces.

The second and subsequent paragraphs need to be indented 4 spaces
from the beginning of the line to be included in the list item.

The code fragments have to be indented 8 spaces from the beginning
of the line (or 4 spaces from the beginning of the preceding paragraph).

Sublists are marked with a hyphen (-). They follow the rules above
with the difference that their initial indent will be greater
(8 spaces from the beginning of the line).
Are the spaces rendered in markdown or are they just added for clarity in the .ex file?