Clarify that gender-unknown option is gender neutral

Clarify that gender-unknown option is gender neutral

When selecting gender in Special:Preferences, this changes "I prefer
not to say" to "When mentioning you, the software will use gender
neutral words whenever possible". This makes the option a catch-all
for any user that doesn't want to be explicitly identified as male
or female (without suggesting that the user doesn't want to answer
the question). The specific wording is based on a suggestion by
Nikerabbit (see T61643#968290).

Bug: T61643
Change-Id: I0b64573bbf8c7813ebc6f7c128f1046d55c73796