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/lock.hh

@@ -13,14 +13,18 @@ struct UserLock
 {
     virtual ~UserLock() { }
 
-    /* Get the first and last UID. */
+    /**
+     * Get the first and last UID.
+     */
     std::pair<uid_t, uid_t> getUIDRange()
     {
         auto first = getUID();
         return {first, first + getUIDCount() - 1};
     }
 
-    /* Get the first UID. */
+    /**
+     * Get the first UID.
+     */
     virtual uid_t getUID() = 0;
 
     virtual uid_t getUIDCount() = 0;
@@ -30,8 +34,10 @@ struct UserLock
     virtual std::vector<gid_t> getSupplementaryGIDs() = 0;
 };
 
-/* Acquire a user lock for a UID range of size `nrIds`. Note that this
-   may return nullptr if no user is available. */
+/**
+ * Acquire a user lock for a UID range of size `nrIds`. Note that this
+ * may return nullptr if no user is available.
+ */
 std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useUserNamespace);
 
 bool useBuildUsers();