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I am sure ghia or someone else will comment if I am wrong but it appears that the file you have modified is a series of lists...
First choice is...
Arial=arial,helvetica,sans-serif
This offers the choice of Arial and will USE arial if it is available. If it is not available the first fall back is helvetica and the second fall back is sans-serif.
By fall back that means if the first one is not available it will use the first fall back and if that one is not available it will use the second fall back on down the line until it finds a font you have or it will fall back to the browser...
So in the portion that reads...
Courier New=courier new,courier,monospace
The choice of Courier New will actually use the Courier new font if available then if not available will use courier font and lastly if that is not available will use the monospace font.
So lets look at the line you entered...
Tahoma=tahoma,arial, Jameel Noori Nastaleeq, Nafees Web Naskh,helvetica,sans-serif
This line will mention the choice "Tahoma" and if the tahoma font is available it will use it first.. If not then it will use arial, if not it will then use jameel noori nastaleeq, if thats not available then Nafees Web Naskh and lastly if NONE of those are available it will default to sans-serif...
If you WANT to offer Jameel Noori Nastaleeq as a choice you will need a line that reads something like...
Jameel Noori Nastaleeq = Jameel Noori Nastaleeq, arial, sans-serif;
And another line like
Nafees Web Naskh = Nafees Web Naskh, arial, sans-serif;
This should offer the two options as fonts and then if those are not available they will fall back to arial and if that is not available it will choose sans-serif.