Improve the Help text in add-forge-request creation page

There are multiple language issues in the Help text. In-line help text in the form also requires some improvement.

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Migrated from T4087 (view on Phabricator)

added 1 deleted label

changed the description

added priority:Normal label

mentioned in commit swh/devel/swh-web@cdda03c3

added state:wip label

assigned to @jayeshv

removed state:wip label

closed