add devtag docs for rel_try()

[maelle]

From the contributing guide

All verbs wrap the code in a rel_try({}) call and fall back to dplyr in the case of failure.
The rel_try() function takes named arguments that describe conditions for an early drop-out, and the corresponding error message.

[maelle]

Maybe align those

• "Implemented for all cases?" = FALSE (means no fallback if I remember correctly?)
• "No restrictions" = FALSE