WIP: Fix GHC.Stack.Types documentation of CallStack to match user's manual (#17858)

This patch to libraries/base/GHC/Stack/Types.hs makes the following improvements to documentation:

• 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).