improve test_docs_coverage error messages (#21029)
Summary:
Most important fix: Correct "tensor.rst" to "tensors.rst"
Secondary fix: some minor English spelling/grammar fixes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21029
Differential Revision: D15523230
Pulled By: umanwizard
fbshipit-source-id: 6052d8609c86efa41a4289cd3a099b2f1037c810
