adds PHPDoc @return and @param for getters and setters respectively (#3131)

* adds PHPDoc @return and @param for getters and setters respectively

* addresses changes in PR review

* adds documentation tests

* Update php_generator:

- Prepend \ to names where required
- Remove <pre> tags
- Update protobuf field comments

* Updates class files with the protobuf changes

* Addresses review comments

* removes Protobuf Type line from PHP generated classes

* fixes phpdoc test

* adds array types to phpdoc
diff --git a/php/phpunit.xml b/php/phpunit.xml
index 0191a60..7cb1e2a 100644
--- a/php/phpunit.xml
+++ b/php/phpunit.xml
@@ -7,6 +7,7 @@
       <file>tests/array_test.php</file>
       <file>tests/encode_decode_test.php</file>
       <file>tests/generated_class_test.php</file>
+      <file>tests/generated_phpdoc_test.php</file>
       <file>tests/map_field_test.php</file>
       <file>tests/well_known_test.php</file>
     </testsuite>