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-accessor.hh

@@ -10,24 +10,30 @@ namespace nix {
 
 struct Source;
 
-/* Return an object that provides access to the contents of a NAR
-   file. */
+/**
+ * Return an object that provides access to the contents of a NAR
+ * file.
+ */
 ref<FSAccessor> makeNarAccessor(std::string && nar);
 
 ref<FSAccessor> makeNarAccessor(Source & source);
 
-/* Create a NAR accessor from a NAR listing (in the format produced by
-   listNar()). The callback getNarBytes(offset, length) is used by the
-   readFile() method of the accessor to get the contents of files
-   inside the NAR. */
+/**
+ * Create a NAR accessor from a NAR listing (in the format produced by
+ * listNar()). The callback getNarBytes(offset, length) is used by the
+ * readFile() method of the accessor to get the contents of files
+ * inside the NAR.
+ */
 typedef std::function<std::string(uint64_t, uint64_t)> GetNarBytes;
 
 ref<FSAccessor> makeLazyNarAccessor(
     const std::string & listing,
     GetNarBytes getNarBytes);
 
-/* Write a JSON representation of the contents of a NAR (except file
-   contents). */
+/**
+ * Write a JSON representation of the contents of a NAR (except file
+ * contents).
+ */
 nlohmann::json listNar(ref<FSAccessor> accessor, const Path & path, bool recurse);
 
 }