docs: note direct_pyi_files/transitive_pyi_files are usually build-time only (#2555)

The pyi_srcs and pyi_deps attributes have this disclaimer already. The
provider should, too, so behavior is better specified.

python/private/py_info.bzl

diff --git a/python/private/py_info.bzl b/python/private/py_info.bzl
index 2a02685..ef654c3 100644
--- a/python/private/py_info.bzl
+++ b/python/private/py_info.bzl
@@ -112,6 +112,9 @@
 this target. Usually they describe the source files listed in
 `direct_original_sources`. This field is primarily for static analysis tools.
 
+These files are _usually_ build-time only and not included as part of a runnable
+program.
+
 :::{note}
 This may contain implementation-specific file types specific to a particular
 type checker.
@@ -190,6 +193,9 @@
 Usually they describe the source files listed in `transitive_original_sources`.
 This field is primarily for static analysis tools.
 
+These files are _usually_ build-time only and not included as part of a runnable
+program.
+
 :::{note}
 This may contain implementation-specific file types specific to a particular
 type checker.