[LangRef] add explanatory text for select poison semantics (PR20895)

This is copied from the suggested text by @regehr in:
https://bugs.llvm.org/show_bug.cgi?id=20895

The way forward was not clear for several years, but now that we
have 'freeze' and Alive2, the behavior should be documented.
Also see comments in D76332.

llvm/docs/LangRef.rst

@@ -3478,9 +3478,12 @@ the ``nsw`` flag.
 
 Poison value behavior is defined in terms of value *dependence*:
 
--  Values other than :ref:`phi <i_phi>` nodes depend on their operands.
+-  Values other than :ref:`phi <i_phi>` nodes and :ref:`select <i_select>'
+   instructions depend on their operands.
 -  :ref:`Phi <i_phi>` nodes depend on the operand corresponding to
    their dynamic predecessor basic block.
+-  Select instructions depend on their condition operand and their
+   select operand.
 -  Function arguments depend on the corresponding actual argument values
    in the dynamic callers of their functions.
 -  :ref:`Call <i_call>` instructions depend on the :ref:`ret <i_ret>`