Add support for overriding description (and summary) for `path` and `ToSchema` macros

[clechasseur]

utoipa derive macros as well as #[utoipa::path] use doc comments for OpenAPI description (and, in the case of path, also summary). For IntoResponses and ToResponse, it is possible to override this via a description attribute, but not for others.

I would suggest adding support for such a description (and summary) overrides for two other macros:

• path
• ToSchema

(It could also be possible to override field-level descriptions, although I see this as less useful personally.)

I volunteer to submit a PR for this if this is deemed an acceptable feature.

(This references #781 and #439; I wanted to open a fresh new issue since I volunteer, so that it has its own tracking issue.)