This makes the somewhat-dubious assumption that every plugin will have uniquely-
named documentation files, instead of automatically putting every plugin's docs
into a namespace. However, plugins already have to deal with flat namespaces for
commands, options, filetypes, etc. so one more shouldn't hurt.
Fixes#2466.
Similarly to the <semicolon> key, make it easier to write
`:execute-keys` commands by replacing <percent> with `%`.
Highlighters can keep escaping the sign when regular expressions are
not quoted, but built-in scripts that use `%` as an editing primitive
have been modified to use the named key, for clarity.
