Clarify that `code` parameter to `/oauth/token` is only required for authorization code grant flow

[ThisIsMissEm]

This currently isn't documented well. The code parameter is not used for client_credentials grant flow.

[mjankowski]

In this section of the methods/oauth page - https://github.com/mastodon/documentation/blob/main/content/en/methods/oauth.md?plain=1#L115-L119 - there is language in the grant type section about using authorization_code only when code is provided.

Is the issue/request here basically to add the inverse clarification to that next section? IE, in the code form parameters description add something like "only required with authorization_code grant flow" or something?

There are multiple spots throughout docs referring to the various oauth/apps/flows/etc - that seems most similar/relevant, but want to be sure.

[ThisIsMissEm]

Not quite, but this improves it: #1796