pw_digital_io: Doxygenify the provides_* methods

Change-Id: Ide224aa87374823751e8a0f5086c9c98d3bb5e03
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/153192
Reviewed-by: Kayce Basques <kayce@google.com>
Reviewed-by: Anton Markov <amarkov@google.com>
Pigweed-Auto-Submit: Kayce Basques <kayce@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>

pw_digital_io/docs.rst

@@ -279,6 +279,7 @@ API reference
    This API reference is incomplete.
 
 .. doxygenclass:: pw::digital_io::DigitalIoOptional
+   :members:
 
 ------------
 Dependencies

pw_digital_io/public/pw_digital_io/digital_io.h

@@ -70,11 +70,11 @@ class DigitalIoOptional {
  public:
   virtual ~DigitalIoOptional() = default;
 
-  // True if input (getting state) is supported.
+  /// @returns `true` if input (getting state) is supported.
   constexpr bool provides_input() const { return config_.input; }
-  // True if output (setting state) is supported.
+  /// @returns `true` if output (setting state) is supported.
   constexpr bool provides_output() const { return config_.output; }
-  // True if interrupt handlers can be registered.
+  /// @returns `true` if interrupt handlers can be registered.
   constexpr bool provides_interrupt() const { return config_.interrupt; }
 
   // Get the state of the line.