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/local-fs-store.hh

@@ -48,7 +48,9 @@ public:
     void narFromPath(const StorePath & path, Sink & sink) override;
     ref<FSAccessor> getFSAccessor() override;
 
-    /* Register a permanent GC root. */
+    /**
+     * Register a permanent GC root.
+     */
     Path addPermRoot(const StorePath & storePath, const Path & gcRoot);
 
     virtual Path getRealStoreDir() { return realStoreDir; }