Merge 'passwordreset-pretext' and 'passwordreset-text' messages

It is confusing to have in the same two messages which can be interpreted
with different meanings. When there is more than one way of resetting a
password, two messages are being show to the user:
- 'passwordreset-pretext' saying to fill only one of the fields below
- 'passwordreset-text' saying to *complete* the form, which people
  can interpret as "fill all the fields below"

This already got changed at the English Wikipedia, see
https://en.wikipedia.org/wiki/Wikipedia:MediaWiki_messages/Archive_5#Special:PasswordReset_improvements
I think it's worth changing MediaWiki too.

Change-Id: Ia2f0c889959d11396411ba44ae595ffad5d498d2
Merge "Merge 'passwordreset-pretext' and 'passwordreset-text' messages"