Consider more clearly communicating that eval tools can only check a few things

[shawna-slh]

In Selecting Eval Tools, make it more clear that tools can only automatically check a small % of WCAG requirements. Probably includes in the summary at the top.

At the top of the Eval Tools List, consider adding brief info about this.

(related to https://github.com/w3c/wai-team/issues/353 (W3C-staff only))