PHPDoc: Replace non-existing return type const with string(|int)

Summary: `const` is not a valid type. See https://docs.phpdoc.org/guide/references/phpdoc/types.html and https://docs.phpdoc.org/guide/references/phpdoc/tags/return.html.

Test Plan: Read docs; run static code analysis.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D26068

src/applications/transactions/storage/PhabricatorModularTransactionType.php

@@ -431,7 +431,7 @@ abstract class PhabricatorModularTransactionType
    * @param object $object Object being edited.
    * @param PhabricatorApplicationTransaction $xaction Transaction being
    *    applied.
-   * @return null|const|list<const> A capability constant (or list of
+   * @return null|string|list<string> A capability constant (or list of
    *    capability constants) which the actor must have on the object. You can
    *    return `null` as a shorthand for "no capabilities are required".
    */

src/infrastructure/customfield/field/PhabricatorCustomField.php

@@ -1554,7 +1554,7 @@ abstract class PhabricatorCustomField extends Phobject {
   /**
    * Get the available conditions for this field in Herald.
    *
-   * @return list<const> List of Herald condition constants.
+   * @return list<string> List of Herald condition constants.
    * @task herald
    */
   public function getHeraldFieldConditions() {

src/infrastructure/edges/editor/PhabricatorEdgeEditor.php

@@ -354,7 +354,8 @@ final class PhabricatorEdgeEditor extends Phobject {
    * Get a list of all edge types which are being added, and which we should
    * prevent cycles on.
    *
-   * @return list<const> List of edge types which should have cycles prevented.
+   * @return list<int|string> List of edge type constants which should have
+   *   cycles prevented.
    * @task cycle
    */
   private function getPreventCyclesEdgeTypes() {