[FW][IMP] stripe: clarify the conditions for the support of a payment method

[fw-bot]

Even though the documentation was correct, it was confusing the reader
to state that local payment methods are considered enabled if their
related payment icon record does not exist in the database. The intended
meaning was that this statement only holds if the local payment method
is also supported, otherwise it cannot be enabled.

Note: the content structure is partly inspired by that in 15.0+ to ease the forward-port.

Forward-Port-Of: #2543

[robodoo]

Pull request status dashboard.

[fw-bot]

@AntoineVDV cherrypicking of pull request #2543 failed.

stderr:

15:52:03.073753 git.c:344               trace: built-in: git cherry-pick 6417fa892111ca6e988b0bbe0ed4b5bd403cba96
error: could not apply 6417fa89... [IMP] stripe: clarify the conditions for the support of a payment method
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add <paths>' or 'git rm <paths>'
hint: and commit the result with 'git commit'
----------
status:

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

[AntoineVDV]

@robodoo r+