Update GridDiag docs

[dwfncar]

Describe the Task

Update GridDiag documentation to clarify behavior when the
-data command line option is used twice.
e.g. -data fileA -data fileB.

Also confirm that the existing grid_diag unit tests include an example of using -data multiple times to specify different input datasets.

Time Estimate

4 hours.

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Relevant Deadlines

MET 10.0.0

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None

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  David Fillmore
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[dwfncar]

In a config data block with varA and varB.
-data fileA -data fileB can be used to have varA read from fileA and varB read from fileB.

[dwfncar]

Merged to dev.