Clarify what commenting *.pubxml does

https://github.com/github/gitignore/commit/08def965cd1c0241a7cffbe7c26a74b3cab61243 incorrect states that commenting *.pubxml will keep your passwords hidden when it will actually do the opposite. This clarifies what commenting *.pubxml does and warns you of the dangers.
database connection strings unencrypted

user passwords are stored in *.user files so it is only database connection strings that are not protected.
Merge pull request #1136 from bbodenmiller/patch-3

Clarify what commenting *.pubxml does