Add doc comments to all crates

[tcr]

Our generated crates on docs.rs need much more documentation, in the form of crate and module-level /// comments. We should flesh these out so users can consult the docs without having to read the source.

• tessel crate
• accel-mma84 crate
• ble crate
• climate-si7020 crate
• relay-mono crate
• servo-pca9685 crate