docs: clarify feed and agency language codes use BCP 47 #1987

[AzazelSensei]

Summary:

The mismatch notice help text said mul comes from ISO 639-2. GTFS language fields are BCP 47, so I updated that wording on the notice and the validator class.

Fixes #1987

Expected behavior:

Help text for feed_info_lang_and_agency_lang_mismatch now says language codes follow BCP 47.

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[skalexch]

@emmambd I raised a related issue here.