Document writing/clear_prefix behavior w.r.t. child tries

[tomaka]

It should be documented that:

• Any call to ext_storage_set_version_1 with a key that starts with :child_storage: is silently ignored.
• Any call to ext_storage_clear_prefix (I guess both versions?) with a key that "clashes" with :child_storage: is silently ignored.

"Clashes" means that for example clear_prefix(":"), clear_prefix(":child_sto"), or clear_prefix(":child_storage:foobar") are all silently ignored, but clear_prefix(":foo") is not ignored.

References:

• For storage_set: https://github.com/paritytech/substrate/blob/c8653447fc8ef8d95a92fe164c96dffb37919e85/primitives/state-machine/src/ext.rs#L392
• For clear_prefix: https://github.com/tomaka/polkadot/blob/5ae005c244295d23586c93da43148b2bc826b137/primitives/state-machine/src/ext.rs#L470-L476 and https://github.com/tomaka/polkadot/blob/5ae005c244295d23586c93da43148b2bc826b137/primitives/storage/src/lib.rs#L229-L235