[clang docs] Rescue some deleted bits of the command-line reference.

Back when the command-line reference rst was in-tree, a lot of people missed
the "DO NOT EDIT" comment at the top, and then changes were
effectively reverted when the file was regenerated. I went through the
changes, and rescued the interesting bits of documentation that were
destroyed.

Additional notes:

- I'm intentionally leaving out D73459 because I'm not sure how to port
  the changes to -march.
- Some options have help text in Options.td, but that text doesn't make
  it into the reference. Incomplete list of such options:
  -fc++-static-destructors, -frtti-data, -fplt, -fstrict-return,
  -funique-section-names, -fuse-init-array.  Not sure what's happening.

Differential Revision: https://reviews.llvm.org/D152396