keycaps: Downgrade "guimenu used to mark up keycap" to warning
keycaps: Downgrade "guimenu used to mark up keycap" to warning

This has a relatively high number of false positives these days, let's
avoid confusing authors.

sknorr/suse-doc-style-checker,sknorr/suse-doc-style-checker,sknorr/suse-doc-style-checker