Finish converting existing comments for internal API docs (#8146)

* Finish converting existing comments for internal API docs

99% of this was just reformatting existing comments. Only two exceptions:

- Expanded upon `BuildResult::status` compat note

- Split up file-level `symbol-table.hh` doc comments to get
  per-definition docs

Also fixed a few whitespace goofs, turning leading tabs to spaces and
removing trailing spaces.

Picking up from #8133

* Fix two things from comments

* Use triple-backtick not indent for `dumpPath`

* Convert GNU-style `\`..'` quotes to markdown style in API docs

This will render correctly.

src/libexpr/eval-inline.hh

@@ -5,7 +5,9 @@
 
 namespace nix {
 
-/* Note: Various places expect the allocated memory to be zeroed. */
+/**
+ * Note: Various places expect the allocated memory to be zeroed.
+ */
 [[gnu::always_inline]]
 inline void * allocBytes(size_t n)
 {