Looking at for the relevant source code, and then checking the git blame information to find where this line came from, provides some context:
I guess they donāt exactly explain the meaning of āthere is very little structured metadataā there eitherā¦ so let me provide some additional context/information that Iām aware of.
Basically, to explain a crateās feature flags for their users, the common approach is simply to explain them in English words, or whatever form they feel appropriate, somewhere in the documentation of the crate. Often, thatās going to be a section of the crateās top-level module documentation (the //!-style doc comments at the top of the main file); and/or in the crateās readme.md file). And sometimes there are also some relevant # comments added around the feature flagsā declarations right in in the Cargo.toml.
However, all in all there isnāt anything like a notion of ādoc commentsā for Cargo.toml files. So there isnāt any official / intended way to specifically generate ādocumentation for feature flagsā from something like that. Nor is there a way to mark e.g. a section of the top-level module doc comments as the ādocumentation for the feature flagsā in a standardized, machine-readable way. So with this current situation, no tool can reliably find ādocumentation for feature flag xyzā for every crate, which is why this ātoolā thatās part of docs.rs for generating this overview/list of all the feature flags only creates a very minimalistic page.
I assume thatās all that the āthere is very little structured metadataā remark is supposed to means.
Auto-generating such an overview is still useful though since you can rely on it to actually accurately list all the feature flags that there are, even when the crate author might have missed documentation for some (or all) feature flags, and/or when that documentation might have gone out-of-date.
AFAICT, the message here has mainly the purpose of convenientely linking to [the docs.rs-hosted versions of] the most common places where you can find out more information about the feature flags. So the second sentence ā with the 3 links ā is probably more important than the first sentence anyway 
In short: "There is currently no way to document feature flags in a way that rustdoc could make this page more useful." The stuff about "structured metadata" is perhaps not the most user-friendly way to phrase it 