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/libexpr/attr-path.hh

@@ -17,7 +17,9 @@ std::pair<Value *, PosIdx> findAlongAttrPath(
     Bindings & autoArgs,
     Value & vIn);
 
-/* Heuristic to find the filename and lineno or a nix value. */
+/**
+ * Heuristic to find the filename and lineno or a nix value.
+ */
 std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what);
 
 std::vector<Symbol> parseAttrPath(EvalState & state, std::string_view s);