Add note about inlining dllimport functions to the attribute docs

[aaron.ballman]

LGTM

clang/include/clang/Basic/AttrDocs.td

def DLLImportDocs : Documentation {
   let Content = [{
 The ``__declspec(dllimport)`` attribute declares a variable, function, or
 Objective-C interface to be imported from an external module. It is available
 under the ``-fdeclspec`` flag for compatibility with various compilers. The
 primary use is for COFF object files which explicitly specify what interfaces
 are imported from external modules. See the dllimport_ documentation on MSDN
 for more information.
 
+Note that a dllimport function may still be inlined, if its definition is
+available and it doesn't reference any non-dllimport functions or global
+variables.
+
 .. _dllimport: https://msdn.microsoft.com/en-us/library/3y1sfaz2.aspx
   }];
 }
 
 def ThreadDocs : Documentation {
   let Category = DocCatVariable;
   let Content = [{
 The ``__declspec(thread)`` attribute declares a variable with thread local