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/ansicolor.hh

@@ -1,9 +1,12 @@
 #pragma once
-///@file
+/**
+ * @file
+ *
+ * @brief Some ANSI escape sequences.
+ */
 
 namespace nix {
 
-/* Some ANSI escape sequences. */
 #define ANSI_NORMAL "\e[0m"
 #define ANSI_BOLD "\e[1m"
 #define ANSI_FAINT "\e[2m"