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/primops.hh

@@ -23,9 +23,11 @@ struct RegisterPrimOp
     typedef std::vector<Info> PrimOps;
     static PrimOps * primOps;
 
-    /* You can register a constant by passing an arity of 0. fun
-       will get called during EvalState initialization, so there
-       may be primops not yet added and builtins is not yet sorted. */
+    /**
+     * You can register a constant by passing an arity of 0. fun
+     * will get called during EvalState initialization, so there
+     * may be primops not yet added and builtins is not yet sorted.
+     */
     RegisterPrimOp(
         std::string name,
         size_t arity,
@@ -38,10 +40,14 @@ struct RegisterPrimOp
    may wish to use them in limited contexts without globally enabling
    them. */
 
-/* Load a ValueInitializer from a DSO and return whatever it initializes */
+/**
+ * Load a ValueInitializer from a DSO and return whatever it initializes
+ */
 void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Value & v);
 
-/* Execute a program and parse its output */
+/**
+ * Execute a program and parse its output
+ */
 void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v);
 
 }