feat: add caveat when relying on Anchor to infer space

[lidatong]

• Add a brief warning section after the existing book section mentioning how to use default in Anchor to infer space.
• This addresses https://github.com/project-serum/anchor/issues/1291 in the main Anchor repo.
• I just learned the hard way :) and hope this can save others time running into the same thing.

[paul-schaaf]

hi @lidatong thanks for the contribution but Im closing this because as armani mentioned, we are removing the default calculation from anchor so this section wont be necessary anymore