Docs for plugin dir like "vim-SOMEPLUGIN" shouldn't silently include "vim-" prefix

[dbarnett]

If there's no plugin name declared in addon-info.json, vimdoc takes the name of the directory that contains the plugin as the plugin name. Often this directory will have a "vim-" prefix or some other variation on the plugin name, which usually should not be taken as the literal plugin name.

Vimdoc should either automatically drop known prefixes/suffixes like "vim-" (the same way maktaba does) or warn when it detects docs being generated for ambiguous plugin names.

Specifying a literal plugin name in addon-info.json should always get you exactly the name you specify with no errors or warnings.