Clarify requirements for relative paths in npm publish.

This adds clarification on relative path support directly to the npm
publish documentation. Currently, this documentation only alludes to
package spec in the "see also" section, leaving it unclear that the
unprefixed relative paths that are supported by most other tools will
fail here. Enhancing this documentation is important since unprefixed
relative paths result in surprising error messages that do not hint at
the appropriate solution.

## References
Related to #2796