docs: mutability of List/Map/Message values in Get and Set

[fdymylja]

The protoreflect.Message interface documentation does not specify how mutabilty of protoreflect.Value for Message, List and Map types works when Getting them or Setting them.

For example, something that I found inconsistent was the fact that if you Set a MapValue or MessageValue and you modify it after, the changes are reflected in the message, this does not appear to be the case for ListValue.

[neild]

We're missing a line of documentation on Message.Set. List.Set, List.Append, and Map.Set all state:

// When setting a composite type, it is unspecified whether the set
// value aliases the source's memory in any way.

Message.Set should have the same text.

In addition, it would be good to have a clear description of aliasing behavior somewhere. The summary is that a protoreflect.Value acquired from a composite value's Get or Mutable method is guaranteed to alias the original value, and that it is otherwise unspecified when aliasing occurs.

So this is a valid way to append to a repeated field:

list := m.Mutable(someRepeatedFieldDesc).List()
list.Append(v) // appends v to the field in m

But this is not:

list := m.NewField(someRepeatedFieldDesc).List()
m.Set(someRepeatedFieldDesc, list)

// It is unspecified whether `list` aliases the field in `m`,
// so the next line may or may not modify m.
list.Append(v)

[neild]

We're missing a line of documentation on Message.Set. List.Set, List.Append, and Map.Set all state:

Actually, this line is there. No idea how I missed it.