[PHP] Reference documentation for classes in PHP package

[bshaffer]

We'd like to have reference documentation for the protobuf classes in PHP's google/protobuf package. The idea is to link to them from our own reference documentation on cloud.google.com. Some examples are Google\Protobuf\Internal\Message and Google\Protobuf\Internal\RepeatedField, which show up in our public API quite often (despite Internal being in the class name).

The existing "reference" documentation is more of a tutorial, and does not contain the class reference docs for the methods / return types / etc that we're looking for. Some of the other languages (Dart, Go, Java, Python) have a section for "API" which contains these reference docs.

We could generate the docs and have them hosted at developers.google.com like the other language reference docs (see below) using Doctum like we do with google/gax and google/common-protos-php

Other language references:

• C# API: https://developers.google.com/protocol-buffers/docs/reference/csharp
• C++ API: https://developers.google.com/protocol-buffers/docs/reference/cpp
• Python API: https://googleapis.dev/python/protobuf/latest/
• Kotlin API: https://developers.google.com/protocol-buffers/docs/reference/kotlin
• Java API: https://developers.google.com/protocol-buffers/docs/reference/java
• Go API: https://pkg.go.dev/google.golang.org/protobuf/proto
• Dart API: https://pub.dev/documentation/protobuf/latest/

[bshaffer]

A second part of this is that PHP is not listed on the protobuf homepage for protobuf, and is instead buried inside "Reference"

https://developers.google.com/protocol-buffers