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/log-store.hh

@@ -10,8 +10,10 @@ struct LogStore : public virtual Store
 {
     inline static std::string operationName = "Build log storage and retrieval";
 
-    /* Return the build log of the specified store path, if available,
-       or null otherwise. */
+    /**
+     * Return the build log of the specified store path, if available,
+     * or null otherwise.
+     */
     std::optional<std::string> getBuildLog(const StorePath & path);
 
     virtual std::optional<std::string> getBuildLogExact(const StorePath & path) = 0;