Add a comment about writableOps_tag

Change-Id: Ib667f21e7ec817ababb8498244a50fd434905796
diff --git a/src/path_parser.hpp b/src/path_parser.hpp
index b3e9418..e1982f5 100644
--- a/src/path_parser.hpp
+++ b/src/path_parser.hpp
@@ -397,6 +397,9 @@
     return schema.isConfig(path);
 };
 
+// A WritableOps value is injected through the `x3::with` with this tag (see usage of the tag). It controls whether
+// `config: false` data can be set with the `set` command. This is used by yang-cli because that tool needs modeling of
+// the full datastore, including the "read-only" data.
 struct writableOps_tag;
 
 PathParser<PathParserMode::DataPath, CompletionMode::Data> const dataPathFilterConfigFalse{filterConfigFalse};