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/value/context.hh

@@ -28,34 +28,37 @@ public:
 
 class Store;
 
-/* Plain opaque path to some store object.
-
-   Encoded as just the path: ‘<path>’.
-*/
+/**
+ * Plain opaque path to some store object.
+ *
+ * Encoded as just the path: ‘<path>’.
+ */
 struct NixStringContextElem_Opaque {
     StorePath path;
 
     GENERATE_CMP(NixStringContextElem_Opaque, me->path);
 };
 
-/* Path to a derivation and its entire build closure.
-
-   The path doesn't just refer to derivation itself and its closure, but
-   also all outputs of all derivations in that closure (including the
-   root derivation).
-
-   Encoded in the form ‘=<drvPath>’.
-*/
+/**
+ * Path to a derivation and its entire build closure.
+ *
+ * The path doesn't just refer to derivation itself and its closure, but
+ * also all outputs of all derivations in that closure (including the
+ * root derivation).
+ *
+ * Encoded in the form ‘=<drvPath>’.
+ */
 struct NixStringContextElem_DrvDeep {
     StorePath drvPath;
 
     GENERATE_CMP(NixStringContextElem_DrvDeep, me->drvPath);
 };
 
-/* Derivation output.
-
-   Encoded in the form ‘!<output>!<drvPath>’.
-*/
+/**
+ * Derivation output.
+ *
+ * Encoded in the form ‘!<output>!<drvPath>’.
+ */
 struct NixStringContextElem_Built {
     StorePath drvPath;
     std::string output;
@@ -84,11 +87,12 @@ struct NixStringContextElem : _NixStringContextElem_Raw {
         return static_cast<Raw &>(*this);
     }
 
-    /* Decode a context string, one of:
-       - ‘<path>’
-       - ‘=<path>’
-       - ‘!<name>!<path>’
-      */
+    /**
+     * Decode a context string, one of:
+     * - ‘<path>’
+     * - ‘=<path>’
+     * - ‘!<name>!<path>’
+     */
     static NixStringContextElem parse(const Store & store, std::string_view s);
     std::string to_string(const Store & store) const;
 };