LangRef: note that `allockind("free")` requires void return

[durin42]

Summary

Otherwise we have to work pretty hard to ensure a discarded alloc/free
pair doesn't remove a return value that's still useful.

[nlopes]

LGTM, thank you!

[nikic]

We should add a corresponding check to the IR verifier.

We should probably also specify that alloc/realloc must return a pointer type.

llvm/docs/LangRef.rst

``allockind("KIND")``
 
     * "alloc": the function returns a new block of memory or null.
     * "realloc": the function returns a new block of memory or null. If the
       result is non-null the memory contents from the start of the block up to
       the smaller of the original allocation size and the new allocation size
       will match that of the ``allocptr`` argument and the ``allocptr``
       argument is invalidated, even if the function returns the same address.
     * "free": the function frees the block of memory specified by ``allocptr``.
+      Functions marked as "free" ``allockind`` must return void.
     * "uninitialized": Any newly-allocated memory (either a new block from
       a "alloc" function or the enlarged capacity from a "realloc" function)
       will be uninitialized.
     * "zeroed": Any newly-allocated memory (either a new block from a "alloc"
       function or the enlarged capacity from a "realloc" function) will be
       zeroed.
     * "aligned": the function returns memory aligned according to the
       ``allocalign`` parameter.