docs: add missing id elements for developer GUCs

Hi

A few of the developer option GUCs were missing the "id" attribute
in their markup, making it impossible to link to them directly.

Specifically the entries from "trace_locks" to "log_btree_build_stats" here:

    https://www.postgresql.org/docs/current/runtime-config-developer.html

Patch applies cleanly back to REL_11_STABLE.

Regards

Ian Barwick