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/libutil/canon-path.hh

@@ -192,8 +192,10 @@ public:
      */
     bool isAllowed(const std::set<CanonPath> & allowed) const;
 
-    /* Return a representation `x` of `path` relative to `this`, i.e.
-       `CanonPath(this.makeRelative(x), this) == path`. */
+    /**
+     * Return a representation `x` of `path` relative to `this`, i.e.
+     * `CanonPath(this.makeRelative(x), this) == path`.
+     */
     std::string makeRelative(const CanonPath & path) const;
 };