[mlir][Transforms] Dialect Conversion: Simplify block conversion API

This commit simplifies and improves documentation for the part of the `ConversionPatternRewriter` API that deals with signature conversions.

There are now two public functions for signature conversion:
* `applySignatureConversion` converts a single block signature.
* `convertRegionTypes` converts all block signatures of a region.

Note: `convertRegionTypes` could be renamed to `applySignatureConversion` (overload) in the future.

Also clarify when a type converter and/or signature conversion object is needed and for what purpose.

From a functional perspective, this change is NFC. However, the public API changes, thus not marking as NFC.