Documentation: deprecate_soft and quite deprecation messages

[bms63]

TB updated: But notes should be placed in documentation around these new functions and packages for deprecation.

Should we also have this practice in admiral?

If we want to have it, we should update the "Deprecation" section in the Programming Strategy.

okay there is also this use of deprecate_soft that is nice as well. I will make an issue

Originally posted by @bms63 in #447 (comment)

[ddsjoberg]

The "official" deprecation process recommended by the tidyverse devs, is to first soft deprecate (aka only message devs about upcoming changes, so they can begin to update their packages so users never know about upstream changes that don't affect them), warning (all users see, and code still works), and finally defunct (function still exported, but errors out).

The soft deprecation is perfect for functions from admiraldev (and potentially skip some of the other steps even), because as the pkg names suggests, these are developer functions.

Regarding adding this strategy for the user-facing functions, I am in favor of adopting the full three-step deprecation process. But would be cautious about using soft deprecations in place of any of the other steps for user-facing functions.