* docs: clarify write_source_files output constraint
For both the "write_source_files" rule's "files" attribute and the
"write_source_file" rule's "out_file" attribute, we ensure that the
destination files and directories to which we'll write sit within the
same containing Bazel package as the Bazel target requesting this
writing. Clarify the documentation for each attribute to make it clear
that we're constraining the destinations and not the sources of the
content to be written.
* Update lib/write_source_files.bzl
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Co-authored-by: Derek Cormier <derek@aspect.dev>
