`CallStack` documentation in `libraries/base/GHC/Stack/Types.hs` is inaccurate

Summary

In commit 7407a66d, GHC's CallStack implementation was modified to never infer HasCallStack constraints. The documentation in the user's guide was updated, but the documentation in libraries/base/GHC/Stack/Types.hs was not.

Proposed improvements or changes

A patch to follow will address the following issues in Types.hs:

• change the documentation of HasCallStack solving to match the correct version in the user's guide
• remove documentation of the CallStack's internal representation in favor of a pointer to the getCallStack function, also based on the explanation in the user's guide
• delete a spurious note about the ?callStack implicit parameter being an implementation detail, as it's already documented elsewhere in the module
• add a bit of detail to getCallStack to explain what the String and SrcLoc are (as it's explained in the user's guide reference to getCallStack).