Module credspropagation would benefit from defining keys as constants

[s3rj1k]

In credspropagation, for different/supported providers would be nice to have string fields in maps as constants with appropriate documentation links (like provider schema docs) for each parameter where available.

[zerospiel]

I believe it might be enough to leave doc links within each symbol's body, without making each key an external constant (most of them are self-describing), it multiplies the number of lines x2, as well as attempts to put links on each from the params having the only link to the corresponding documentation page. @a13x5 WDYT?