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/libstore/pathlocks.hh

@@ -5,12 +5,16 @@
 
 namespace nix {
 
-/* Open (possibly create) a lock file and return the file descriptor.
-   -1 is returned if create is false and the lock could not be opened
-   because it doesn't exist.  Any other error throws an exception. */
+/**
+ * Open (possibly create) a lock file and return the file descriptor.
+ * -1 is returned if create is false and the lock could not be opened
+ * because it doesn't exist.  Any other error throws an exception.
+ */
 AutoCloseFD openLockFile(const Path & path, bool create);
 
-/* Delete an open lock file. */
+/**
+ * Delete an open lock file.
+ */
 void deleteLockFile(const Path & path, int fd);
 
 enum LockType { ltRead, ltWrite, ltNone };