[DOCS] Clarify expected diagram properties to fix #233

[ulupo]

Reference issues/PRs Fixes #233.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description As observed in #233, there are implicit assumptions on the input diagrams to the fit and transform methods of most transformers in gtda/diagrams. Most stem from the fact that we call the _subdiagrams function which will break unless all sample diagrams in a collection X have the same number of persistence triples in each homology dimension. This PR extends the approach introduced in https://github.com/giotto-ai/giotto-tda/pull/436/commits/9068c9d978d87ad4279a89fc2a9f26bb6ceea1b7 for Filtering to all transformers in gtda/diagrams with the only exception of ForgetDimension (which does not make use of _subdiagrams).

Checklist

• [x] I have read the guidelines for contributing.
• [x] My code follows the code style of this project. I used flake8 to check my Python changes.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed. I used pytest to check this on Python tests.