[REQ] Documentation a detail about selective generation of apis

Is your feature request related to a problem? Please describe.

I tried to generate code of specific api. I found that there is no detail document about selective generation of apis and then
I read the related codes. I think that future users like me would like to found the solution about selective generation of apis only reading document. A problem is that reading codes to use tools is undesired.

Describe the solution you'd like

I read the related codes about selecting apis.

• Select api from paths map key
• Paths map key is equal with operations map key and tag name
• Default tag name
• Sanitize tag name

The solution I'd like is to add descriptions that users need to set tag name to apis key. (e.g. --global-property apis="Pet,User") and if possible add more description about sanitize rules of tag name.

Describe alternatives you've considered

Additional context

Start with the linked sections of DefaultGenerator.java and DefaultCodegen.java to confirm how selective API generation uses paths, operation keys, tag names, and sanitization. Update the relevant user documentation to explain setting --global-property apis="Pet,User" and, if possible, the tag-name sanitization rules; done means users can configure selective generation without reading source code.