RFC: Deprecate RegField without a RegFieldDesc?

**Type of issue**: other enhancement

**Impact**: API modification

**Development Phase**: proposal

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**

**What is the current behavior?**

Currently, when using the `regmapper` library, one can implement a `RegField` without any description. The associated `RegFieldDesc` can be used to automatically generate downstream information like documentation, header files, other formats such as SVD/IP-XACT, etc. But only if people add the descriptions.

**What is the expected behavior?**

The proposal is to deprecate the RegField functions that use the default of `None` for the RegFieldDesc. So that people get deprecation warnings if they are implementing `regmapper`s without `RegFieldDesc` included.

**Please tell us about your environment:**

**What is the use case for changing the behavior?**

Requiring designers to auto-document their design in-the-code, vs chasing them after the fact.