Clarify pstring draft

[mxrth]

Currently the pstring draft does not specify where pstrings must/should be used, it just says that there are pstrings and that pstrings are a good thing. This led to some confusion because the whole draft is screaming "you must use pstrings" but it's never explicitly said. Something like "the pstring format must be used to represent strings" should fix this.

[kksym]

I'm a little unsure of whether we have standardised this yet or not. There was a little bit of discussion about string format here, if that's helpful in any way.

[enkore]

The consensus was (more or less) generalized that for everything that goes into a DCPU-16 binary pstrings should be used

[Jarvix]

The voting was suspended due to complains from dylukes. He said pstrings are useless in Clang/LLVM... his words. Ask him.