help: add pad function to template help

Commit aa51392da507 introduced a pad function for use in templates, but did
not add the corresponding documentation to 'hg help templating'.

mercurial/help/templates.txt

@@ -57,6 +57,8 b' In addition to filters, there are some b'
 
 - label(label, expr)
 
+- pad(text, width[, fillchar, right])
+
 - revset(query[, formatargs])
 
 - rstdoc(text, style)