Clarify the Webform interaction field descriptions (Webform, Mode, Submission id variable)

Problem/Motivation

The three field descriptions on the Webform external-interaction step were unclear or misleading. "Mode" did not say what each of the three modes does. "Submission id variable" read "Set it to the name the collect step stores", which is vague, and it implied the variable applies to review only, when in fact every mode uses it (a collect step reopens the party's own submission through it). "Webform" did not mention what the required handler does.

Proposed resolution

Rewrite the three field descriptions to state plainly what the handler does, what each mode does, and that the submission variable identifies the submission the step acts on in every mode: how it matches the collect handler's "Store submission id in variable" setting, and when a distinct name is needed (the same form collected more than once, or parallel branches). Update the French translation to match.

Remaining tasks

Review and merge.