Fix #1369 + #1370: adding GearmanClient::setXXXCallback documentation

[cracksalad]

This PR adds documentation for the GearmanClient::setXXXCallback methods.

This includes:

• specification of arguments and return type of each callback method (see Gearman: setXXXCallback() parameter definition incomplete #1370)
• a note that the setXXXCallback methods should be called before calling addTaskXXX (see Gearman: setXXXCallback() needs to be called before addTaskXXX() #1369)

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In the PECL extension you can see, that all callback functions are called (equally) by this code snippet:

https://github.com/php/pecl-networking-gearman/blob/a52052cdd712a95091ce926be3bcdca41c730696/php_gearman_task.c#L32-L48

If you look closely, you might note, that task->zdata (which is called $context in PHP) will be provided if it has been set when GearmanClient::addTaskXXX was called. As I have tested, otherwise null is passed and no warning/error/exception is raised.

For some of the GearmanClient::setXXXCallback methods, there are already examples in this documentation mentioning, that those two arguments are provided.

[Girgias]

Considering the amount of times this text is repeated, I would recommend defining a new XML entity in language-snippets.ent or use XIncludes. :)

[Girgias]

Probably rephrasing this to:

Suggested change

	It must return either nothing (which defaults to <constant>GEARMAN_SUCCESS</constant>)
	or a valid <link linkend="gearman.constants">Gearman return value</link>.
	It should return a valid <link linkend="gearman.constants">Gearman return value</link>.
	However, if &null; is returned it defaults to <constant>GEARMAN_SUCCESS</constant>.

as void functions do return null.

[cracksalad]

Considering the amount of times this text is repeated, I would recommend defining a new XML entity in language-snippets.ent or use XIncludes. :)

As far as I understand, XIncludes would require relative pathes, which might be error-prone. In addition to that, the language-snippets.ent already contains entities for the Gearman extension. So I have added one there.

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@Girgias If I summarize correctly, your other remark is about the void return type and the invalid union type (I guess "invalid" in this case means, that it is not valid PHP code, although it works fine in the docs). As I do not like the idea of expecting people to know that void functions actually return null, I tried to work around that.

[Girgias]

One comment about the structure of the entity, but LGTM otherwise

[Girgias]

Thank you!