Looking for the reason why man 5 webapp.eclass does not show up, I found that the man page is provided only in xml format. To overcome this, emake had to run "app-admin/webapp-config-1.50.16-r3/doc/Makefile". However, this would need app-text/xmlto" which isn't in the ebuild dependency list. Will attach a patch which adds a use flag "doc", adds a conditioanl dependency on app-text/xmlto and bumps EAPI to 1 -- for being able to set IUSE to "+doc", because without doc you're lost here.
Created attachment 211003 [details, diff] Enable the generation of webapp.eclass.5 man page
Thanks for the patch but it's not the way to go I'm afraid. In -r3 installing of eclass manpage was even purposely removed as it collides with eclass-manpages package.
For webapp.eclass manpage, please emerge eclass-manpages.