crypto/tls: add docs detailing the sequence before/after Read()/Write() during TLS handshake

[martindg]

What version of Go are you using (go version)?

$ go version
go version go1.13.5 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What did you expect to see?

Documentation (e.g. for Listen()) telling that actual TLS handshake would only occur after Read(), Write() or explicit call to Handshake() as described here:

go/src/crypto/tls/conn.go

Line 1324 in c2edcf4

// explicitly: the first Read or Write will call it automatically.

What did you see instead?

Explicit information only in the Handshake() documentation:

go/src/crypto/tls/conn.go

Line 1324 in c2edcf4

// explicitly: the first Read or Write will call it automatically.

[toothrot]

/cc @FiloSottile @katiehockman