Added some <see> tags

Added some <see> tags to the documentation comments, and added a CDATA
where it was necessary but not used.
Fix typo in InitializeNestedTypesMap
Fix typos in XML comments
check for null parameter as base class does, but throw ArgumentException
Merge branch 'typo-md-reader' of https://github.com/jbevain/corefx
XmlAttributeCollection.SetNamedItem(XmlNode) returns null if provided a null parameter
System.Xml.XmlDocument.Tests formatter changes
System.Xml.XmlDocument.Tests Sanitizer changes
Replacing foo:bar with val1:val2
Restoring TODOs in comments
Merge branch 'xmltypo'
Remove unused using statements
Update Developers.md

Fixing the name of and adding a link to Visual Studio Community 2013.
Merge remote-tracking branch 'upstream/master' for SetNamedItemTests
Sort usings.
Merge branch 'remove-unused-usings'
Fix build break due to using being removed that was actually used.
Documentation comments: CDATA to {T} for generics

Replaced CDATA with {T} syntax for generics in documentation comments.
Changed references of (float)Math.PI to use its own Pi property instead.
Fix System.Collections.Immutable!ImmutableArray<T>.Builder.Sort() to not generate Comparer wrapper each time
System.Collections.Immutable!ImmutableArray<T>.Builder.Sort() (3 overloads) creates a new Comparer wrapper each time. This can get expensive if you call Sort a lot.

This fix optmizes method Sort() alone among the 3 overloads. Sort() uses the default comparer which through this fix is getting cached into a static field in type Comparer such that the Comparer wrapper is not created each time Sort() is called.
Existing testcase Sort(), covers this case.