Add help links and descriptions to analyzers

For some of them I didn't had any good description that
wasn't a repetition of the text so I avoided to add it
but still added the help link to all of them.

While reviewing all of them I also added the tag to get
greyed-text on analyzer indicating unnecesary constructs:

    customTags: WellKnownDiagnosticTags.Unnecessary

Follow the discussion in #78
Merge pull request #94 from vbfox/descriptions-and-help-links

Add help links and descriptions to analyzers