[C-METADATA] package.documentation should be optional

[behnam]

The package.documentation now has a reasonable default value, using docs.rs, meaning that it actually makes sense to not have it specifically set for most crates.

Therefore, it makes sense to update C-METADATA to mention this default value and ask the config to be set only if it's not the default docs.rs link. This exception would be similar to the package.homepage config.

[dtolnay]

Thanks! I removed the docs.rs guideline in b7a831e and explained that the documentation metadata in Cargo.toml is optional in 17073b7.