users-guide: Rework pragma key generation
users-guide: Rework pragma key generation

Previously we had a hack to handle the case of multi-token SPECIALISE
pragmas. Now we use a slightly more general rule of using a prefix of
tokens containing only alphabetical characters.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc