Document relationship of WriteSecurity and Y2Users writer

[ancorgs]

This is part of the big modification we are doing in the separate y2users branch in order to ensure the interactive module of YaST2 uses the shadow tools under the hood.

We were not sure whether the UI option "Expert Options -> Password Encryption" would still work after the changes. It does work correctly and this pull request simply adds a comment explaining why.

In short, changing the option takes effect immediately and will affect all the users created after altering the value. Users created in the same YaST execution but before changing the value retain the previous encryption method. The value is finally written to the login.defs configuration just to make it persistent, but not to affect the current execution.

[coveralls]

Pull Request Test Coverage Report for Build 1307460367

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 56.193%

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Totals
Change from base Build 1307082948:	0.0%
Covered Lines:	2849
Relevant Lines:	5070

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💛 - Coveralls