Add comment about using efficient map clear with Go 1.20 as base

[Jacalz]

Description:

Just a quick fix to make sure we are using the fast map delete pattern instead of allocating a new map on clear. This compiles down to basically the same thing as doing clear(m.m) in Go 1.21 or later.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 62.419% (+0.05%) from 62.372%
when pulling 3f4b9bf on Jacalz:efficient-map-clear
into e471a5a on fyne-io:develop.