Create update_editing-units.md

[1jc]

issue OpenEnergyDashboard#906

Description

(Please include a summary of the change and which issue is touched on. Please also include relevant motivation and context.)

Describe the bug
If you change what a unit represents then bad things can happen. A conversion might now exist between different types of units. During creation this is not allowed. This is esp. dangerous if this is the meter graphing unit.

Expected behavior
OED should stop a user from making this type of edit and be told to fix up the dependencies. Similar types of checks happen in group edits.

Fixes #[906]

Type of change

(Check the ones that apply by placing an "x" instead of the space in the [ ] so it becomes [x])

• Note merging this changes the database configuration.
• [x ] This change requires a documentation update

Checklist

(Note what you have done by placing an "x" instead of the space in the [ ] so it becomes [x]. It is hoped you do all of them.)

• [x ] I have followed the OED pull request ideas
• I have removed text in ( ) from the issue request

Limitations

none