Update the description for the 'docs' build option

The `docs` option only controls the build of the API reference; for the
man pages, there's the `man` option.
1 job for json-glib-1-4 in 1 minute and 36 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #106201
build-json-glib

00:01:36