Add some notes to the POD in src/call/context.c to discuss the proper use of  Parrot_pcc_get_*_reg functions.
Add some notes to the POD in src/call/context.c to discuss the proper use of  Parrot_pcc_get_*_reg functions.

These functions are marked PARROT_EXPORT, and they probably need to be for things like dynops. However, we should definitely provide some warnings/guidance in case somebody wants to try to use these functions for other purposes.

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