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/libcmd/editor-for.hh

@@ -5,8 +5,10 @@
 
 namespace nix {
 
-/* Helper function to generate args that invoke $EDITOR on
-   filename:lineno. */
+/**
+ * Helper function to generate args that invoke $EDITOR on
+ * filename:lineno.
+ */
 Strings editorFor(const Path & file, uint32_t line);
 
 }