DOC: Inverse transform sampling accuracy warning

[rrjbca]

Description

Add warnings to the docstrings for schechter and redshifts_from_comoving_density highlighting that the user must choose the resolution and redshift parameters respectively so as to satisfy their own numerical accuracy requirements.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [x] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request