Doc suggestion: point out that getAttachment returns a reference

[DarrenCook]

If you modify the Uint8Array that getAttachment() returns, the data inside the (in-memory) DB gets changed too.

This was unexpected, and I believe it is different to the way normal PouchDB behaves.

I've shown some code below to demonstrate the problem. I think a simple fix is to run it through a Uint8Array constructor.

const blob = new Uint8Array( await db.getAttachment(id, fname) )

I was originally going to say this just needs to be written in the docs... but as I've been writing this up, if the point of the pouchdb-memory adapter is to simulate the real thing, then maybe the above code snippet belongs in getAttachment() ?

const blob = await db.getAttachment(id, fname)
const blob2 = await db.getAttachment(id, fname)
blob === blob2  // true
blob[0] = 77
const blob3 = await db.getAttachment(id, fname)
blob[3] === 77