template.py: improve doc
template.py: improve doc

-subst: substitution not available insied <pagelit ... /> tag.

Bug: T151940
Change-Id: I90f403496f95e3479063ae1393981c4c88993296

Darkdadaah/pywikibot-core,wikimedia/pywikibot-core,hasteur/g13bot_tools_new,npdoty/pywikibot,hasteur/g13bot_tools_new,npdoty/pywikibot,wikimedia/pywikibot-core,jayvdb/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new,happy5214/pywikibot-core,happy5214/pywikibot-core,Darkdadaah/pywikibot-core,jayvdb/pywikibot-core,PersianWikipedia/pywikibot-core,magul/pywikibot-core