docs: correct doc comment for PositionCodec.client_num_units

[RossBencina]

Description (e.g. "Related to ...", etc.)

Fixes an incorrect doc comment that was a hangover from the old utf-16 only Position implementation.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

[tombh]

Thank you