Remove 'message' parameter docs from assert.rst
Remove 'message' parameter docs from assert.rst

As per:

	https://github.com/pytest-dev/pytest/issues/3974#issuecomment-463462732

Also made the 'match' parameter more prominent

Akasurde/pytest,The-Compiler/pytest,nicoddemus/pytest,pytest-dev/pytest,tomviner/pytest,RonnyPfannschmidt/pytest,markshao/pytest,alfredodeza/pytest,nicoddemus/pytest,The-Compiler/pytest,tomviner/pytest