docs: clarify limitations of header decoration

[yaauie]

Two known limitations with headers:

1. Only headers whose values are utf-8 byte sequences will be added to the event (impl).
   While in theory this could change in the future, receiving anything other than a known-constant encoding is problematic for a variety of reasons and likely the reason for the current implementation (e.g., not being embeddable in JSON without loss, which precludes sending the raw bytes to Elasticsearch or other outputs).
2. Kafka allows multiple headers to share a key, but as-implemented we will overwrite a key if we observe and set it twice.

[jsvd]

Content-wise LGTM, not sure if the second note will show up correctly cc @karenzone

[karenzone]

NOTE format is legit, but the first sentence is not grammatically correct. Please consider suggestion.

[karenzone]

LGTM 🚀