Improve auto_wrap_policy documentation

[rohan-varma]

📚 The doc issue

A few improvements can be made to the doc of auto_wrap_policy:

• Add expected type of callable
• Clarify what the policy function should return, and how its used (i.e. what's done depends on recurse flag).

Suggest a potential alternative/fix

No response

cc @pietern @mrshenli @pritamdamania87 @zhaojuanmao @satgera @rohan-varma @gqchen @aazzolini @osalpekar @jiayisuse @SciPioneer @H-Huang @kwen2501