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/nar-info-disk-cache.hh

@@ -43,8 +43,10 @@ public:
         const std::string & uri, const DrvOutput & id) = 0;
 };
 
-/* Return a singleton cache object that can be used concurrently by
-   multiple threads. */
+/**
+ * Return a singleton cache object that can be used concurrently by
+ * multiple threads.
+ */
 ref<NarInfoDiskCache> getNarInfoDiskCache();
 
 ref<NarInfoDiskCache> getTestNarInfoDiskCache(Path dbPath);