Correct variable markup in manuals

Correct variable markup in manuals

* doc/lispref/minibuf.texi (Completion in Buffers): Fix markup.
* doc/lispref/text.texi (Filling): Likewise.
* doc/misc/dired-x.texi (Advanced Mark Commands): Likewise.
* doc/misc/epa.texi (Encrypting/decrypting gpg files): Likewise.
(Encrypting/decrypting gpg files): Likewise.
* doc/misc/tramp.texi (External methods): Likewise.
(Default Method): Likewise.
* doc/misc/viper.texi (Editing in Insert State): Likewise.
(File and Buffer Handling): Likewise.
* doc/misc/gnus.texi (Mail Source Customization): Likewise.
* doc/misc/url.texi (Customization): Likewise.  Note obsolete
status of url-temporary-directory.