Document `platform` and `python` shorthands for long-form dependencies

[neersighted]

Issue Kind

Improving documentation

Existing Link

https://python-poetry.org/docs/dependency-specification/#using-environment-markers

Description

The marker documentation in the dependency spec does not currently document the python and platform short-hands. We should document them, and make clear whether or not they should be used (or are deprecated); the combining semantics with markers also needs to be made clear.

We should also explain why they exist, whether or not future short-hand forms are likely, and provide more examples showcasing how to use the markers field for end users.

[rumitvn]

Opened #10952 documenting the python and platform shorthands in the Using environment markers section — their marker equivalents, that they're not deprecated, the and-combining semantics with an explicit markers value, and the PEP 621 [project] alternative. Scoped to current behavior.