clarify syntax for source file in mbx script

[kcrisman]

This low-priority enhancement request is to clarify exactly when full paths are necessary for mbx.

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The file currently at the author guide says

Certain arguments that are filenames require a full (not relative) path to locate the right objects. In the example above, you can see the source file requires this (where the shell in use here will expand the ~ to the user's home directory). However, perhaps surprisingly, when I use

$ ../../mathbook/script/mbx -vv -c sageplot -f svg -d ../images ../src/NTIC.xml 

everything seems to work fine, even though I get

MBX-DEBUG: CLI args {'xmlid': '', 'data_dir': None, 'verbose': 2, 'format': 'svg', 'component': 'sageplot', 'server': None, 'abort': False, 'xml_file': '../src/NTIC.xml', 'stringparams': None, 'dir': '../images', 'out': None}

and the only two other times this shows up in the debugging info it still says ../src/NTIC.xml, not an expanded version. I don't believe my shell is expanding this, after a bit of experimenting.

[davidfarmer]

obsolete, due to CLI