[Minor] Add some descriptions

rules/bitcoin.lua

 
 rspamd_config:register_symbol{
   name = 'BITCOIN_ADDR',
+  description = 'Message has a valid bitcoin wallet address',
   callback = function(task)
     local rspamd_re = require "rspamd_regexp"
     local hash = require "rspamd_cryptobox_hash"

rules/headers_checks.lua

 local prio_cb_id = rspamd_config:register_symbol {
   name = 'HAS_X_PRIO',
   type = 'callback,mime',
+  description = 'X-Priority check callback rule',
   score = 0.0,
   group = 'headers',
   callback = function (task)