There's reasonable documentation of the yield statement for most python versions under Section 6: Simple Statements, particularly 6.8 "The Yield Statement" (http://docs.python.org/release/2.7/reference/simple_stmts.html#the-yield-statement)

But, there's no mention of the return value of the yield statement, or that the send statement even exists.  It's mentioned in passing here under "PEP342 New Generator Features" (http://docs.python.org/whatsnew/2.5.html#pep-342-new-generator-features) but should be given fuller explanation and cross-linking from the yield statement documentation. 

It's also mentioned a bit here: http://docs.python.org/howto/functional.html#passing-values-into-a-generator but again, not under the language documentation itself.