docs/api: clean up Upload API docs slightly

[woodruffw]

This is an attempt to clean up the Upload API docs (very) slightly, as well as document some other well-known edge cases/discrepancies between the metadata layout and the POST form layout.

Key changes:

• Made formatting more uniform: list items now mostly start with the field they're describing, instead of describing the field in free-form prose.
• Elaborated on the description of how metadata is transformed into form fields, including a description of how multiple-use fields are pluralized (Classifier -> classifiers). Also added a table visualizing the different cases.
• Added a clarifying link about the format of pyversion Python tags.
• Replaced i.e. with e.g. where appropriate.

I think this (along with the other recent upload API doc changes) is sufficient to close #3151 🙂

[di]

I'm not sure if this is exactly true or if there are some edge cases. I haven't looked closely but some fields are getting mapped here while others like classifier/classifiers aren't:

warehouse/warehouse/forklift/metadata.py

Lines 270 to 276 in 80d26fd

	# Map Form fields to RawMetadata
	_override = {
	"platforms": "platform",
	"supported_platforms": "supported_platform",
	"license_files": "license_file",
	}
	_FORM_TO_RAW_MAPPING = {_override.get(k, k): k for k in _RAW_TO_EMAIL_MAPPING}