Editorial: Clarify / make consistent naming steps for HTML form controls

[scottaohara]

This should not contain any normative changes

• Updates the listed inputs and button types to steps more consistent in wording.
• incorporates https://github.com/w3c/accname/issues/246 into the label steps for the mentioned form controls
• starting to migrate from just using "subtree" to "text equivalent computation of the element's subtree" - as the simple use of 'subtree' has come up in a handful of reviews for html aam naming steps (this is new in these steps, but the behavior is already reality)

the "other form controls" is not touched in this update - as there are also other pending PRs - re select element - that start breaking that apart.