docs: doc the imports attribute as a target-relative path (#3571)

The docs incorrectly refer to the path as a repo-root relative path, but
this is
plainly not the case, as the underlying code does, essentially,
`join(repoRoot, ctx.label, importsAttr)`

Work towards #3565

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: rickeylev

python/private/attributes.bzl

@@ -189,7 +189,14 @@ List of import directories to be added to the PYTHONPATH.
 Subject to "Make variable" substitution. These import directories will be added
 for this rule and all rules that depend on it (note: not the rules this rule
 depends on. Each directory will be added to `PYTHONPATH` by `py_binary` rules
-that depend on this rule. The strings are repo-runfiles-root relative,
+that depend on this rule.
+
+The values are target-directory-relative runfiles-root paths. e.g. given target
+`//foo/bar:baz`, `sys.path` will be affected as:
+* `a/b` adds `$runfilesRoot/$repo/foo/bar/a/b`
+* `../sibling` adds `$runfilesRoot/$repo/foo/sibling`
+* `../../` adds `$runfilesRoot/$repo`
+(where `$repo` is the name of the repository containing the target).
 
 Absolute paths (paths that start with `/`) and paths that references a path
 above the execution root are not allowed and will result in an error.