xmlchange: Add documentation for setting values with commas

[jgfouca]

This has come up a few times and confused our users.

Test suite: None, just a docs change Test baseline: Test namelist changes: Test status: bit for bit

User interface changes?:

Update gh-pages html (Y/N)?:

[klindsay28]

Spelling correction from the peanut gallery... alternaltive -> alternative

[jgfouca]

Thanks, @klindsay28 . Fixed.