The official documentation of token.tag_ in spaCy is as follows:
A fine-grained, more detailed tag that represents the word-class and some basic morphological information for the token. These tags are primarily designed to be good features for subsequent models, particularly the syntactic parser. They are language and treebank dependent. The tagger is trained to predict these fine-grained tags, and then a mapping table is used to reduce them to the coarse-grained .pos tags.
But it doesn't list the full available tags and each tag's explanation. Where can I find it?
Answer
Finally I found it inside spaCy's source code: tag_map.json. And this link explains the meaning of different tags.