Clearer docs on config merging

[matthewhughes-uw]

The docs (https://opentelemetry.io/docs/collector/configuration/) mention:

You can provide one or more configurations using the --config option

but don't expand on how the multiple configurations will be merged, e.g. how are duplicate maps in a key handled, are existing values stomped or are new values ignored?

Glancing at the code:

• creating a config calls NewResolver

  opentelemetry-collector/otelcol/configprovider.go

  Line 66 in fa49026

  mr, err := confmap.NewResolver(set.ResolverSettings)

• The Resolve func on that type calls Merge on the Conf object

  opentelemetry-collector/confmap/resolver.go

  Line 173 in fa49026

  if err = retMap.Merge(retCfgMap); err != nil {

• Then Merge on that type

  opentelemetry-collector/confmap/confmap.go

  Line 163 in fa49026

  return l.k.Merge(in.k)

  uses https://pkg.go.dev/github.com/knadh/koanf/v2#Koanf.Merge

Maybe it would be sufficient to link to https://pkg.go.dev/github.com/knadh/koanf/v2#readme-merge-behavior