documentation: Fixed-length types

There is a heading called "Fixed-length-tuples" which talks about "Fixed-length-types". Additionally, it is difficult to make out what the user should take away from this section. The wording may need to be cleaned up and an example may possibly help.

Locate the documentation heading "Fixed-length-tuples" and read the surrounding section. Clarify the terminology and the intended takeaway, then add an example if it makes the distinction easier to understand; done means the heading and section wording consistently describe fixed-length types.

Written by the indexing model from the issue text.