Improve `MirPatch` documentation and naming.

It's currently lacking comments. This commit adds some, which is useful
because there are some methods with non-obvious behaviour.

The commit also renames two things:
- `patch_map` becomes `term_patch_map`, because it's only about
  terminators.
- `is_patched` becomes `is_term_patched`, for the same reason.

(I would guess that originally `MirPatch` only handled terminators, and
then over time it expanded to allow other modifications, but these names
weren't updated.)

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