Make also the corresponding change in cpdef signature resolution.

This is a behavior change, as cross-product specializations are no
longer generated for memoryview arguments.

--HG--
extra : rebase_source : 7b763fbb0f2485056ec4d2f53a5ec76dc7be9203

Fix grammar/typo in build docs

There was a word missing.

Avoid dramatic changes in the resulting .c file when the input .pyx file changes only slightly.