Clarify pytest_assertrepr_compare in Docs

[kevinjfoley]

I think the documentation on the assert page for "Defining your own assertion comparison" could be clarified.

pytest/doc/en/assert.rst

Lines 208 to 212 in 7dcd9bf

	Defining your own assertion comparison
	----------------------------------------------
	It is possible to add your own detailed explanations by implementing
	the ``pytest_assertrepr_compare`` hook.

As is I interpret it as addressing how to control how two elements are compared in an assert statement. I also received an answer to a question on Stack Overflow that suggested someone else interpreted this way so I figured I'd suggest making a change.

I think changing the title to something like "Defining your own explanation for failed assertions" would make things clearer and is more in line with the docstring for pytest_assertrepr_compare.

[nicoddemus]

Hi @kevinjfoley thanks for the suggestion! Would you be willing to open a PR with that?