Document count parameters for commands

Closes #895

README.asciidoc

@@ -739,6 +739,7 @@ Some of Kakoune state is available through environment variables:
  * `kak_timestamp`: timestamp of the current buffer, the timestamp is an
        integer value which is incremented each time the buffer is modified.
  * `kak_runtime`: directory containing the kak binary
+ * `kak_count`: count parameter passed to the command
  * `kak_opt_<name>`: value of option <name>
  * `kak_reg_<r>`: value of register <r>
  * `kak_socket`: filename of session socket (/tmp/kak-<session>)

doc/manpages/expansions.asciidoc

@@ -74,6 +74,8 @@ informations about Kakoune's state:
 	which is incremented each time the buffer is modified
 *kak_runtime*::
 	directory containing the kak binary
+*kak_count*::
+	count parameter passed to the command
 *kak_opt_<name>*::
 	value of option *name*
 *kak_reg_<r>*::