docs: pxar create: update docs to match current behavior

docs: pxar create: update docs to match current behavior

This removes parts of the previous explanation of the tool that are no longer
correct, and adds an explanation of '--exclude' parameter, instead.

Adds more clarity to the command, by use of '/path/to/source' to signify
source directory.

Specify that the pattern matching style of the exclude parameter is that of
gitignore's syntax.