[Docs] Clarify that FunctionPasses can't add/remove declarations

In preparation for potential future concurrency, a FunctionPass
shouldn't modify anything at the module level that other FunctionPasses
can also modify.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D89890

Author: aeubanks

llvm/docs/WritingAnLLVMPass.rst

@@ -474,7 +474,8 @@ external functions.
 To be explicit, ``FunctionPass`` subclasses are not allowed to:
 
 #. Inspect or modify a ``Function`` other than the one currently being processed.
-#. Add or remove ``Function``\ s from the current ``Module``.
+#. Add or remove ``Function``\ s from the current ``Module``, including
+   declarations.
 #. Add or remove global variables from the current ``Module``.
 #. Maintain state across invocations of :ref:`runOnFunction
    <writing-an-llvm-pass-runOnFunction>` (including global data).