Added i18n functions and messages to PageSchemas

Added messages to "PageSchemas.i18n.php":-
1.'ps-createpage-irregulartext'
2.'ps-createpage-invalidtitle'
3.'ps-createpage-notfound'
4.'ps-field-display-set'
and subsequently changed code in PS_CreatePageJob.php and specials/PS_EditSchema.php
Documentation is also provided.

Bug: 48435
Change-Id: If8ea01e256e22d8f932b9071340a62c7ca8e5e9b
Merge "Added i18n functions and messages to PageSchemas"