Cargo: Document how to conditionally enable features in dependencies

[aturon]

The / syntax in the features section appears to be undocumented.

[aep]

for anyone else coming here googling:

[dependencies]
othercrate = "1.2.3"

[features]
myfeature = ["othercrate/otherfeature"]

[dwijnand]

Is it? The [features] section in the manifest reference docs (link) has the example:

# Features can be used to reexport features of other packages. The `session`
# feature of package `awesome` will ensure that the `session` feature of the
# package `cookie` is also enabled.
session = ["cookie/session"]

(which has been there since the feature's inception in #712)

Optimistically closing that this was just missed (I think mdbook search wasn't around in March). If this still needs work we can reopen.

[hvenev]

How does this work if cookie is optional?