[roll third_party/pigweed] pw_digital_io: Document the private virtual API requirements

Adds explicit documentation for the private virtual API requirements
given that it was easy to miss requirements when implementing
concrete implementations of pw_digital_io.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/187669

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: ca5ff7956df3e57..d79c2d390cb54f0
Roller-URL: https://ci.chromium.org/b/8758432180901785441
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I1262673a85a0db07ca6cf2d64ccb2090f2a0a83b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/188420
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index ca5ff79..d79c2d3 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit ca5ff7956df3e57421db4c592663a4ea4b0d5a8e
+Subproject commit d79c2d390cb54f03b845f7fc7f5d73cc58ee7faf