message: docs: note which header expressions cannot be used for request headers
  As discussed in some custom header expressions evaluate as
  empty when used in request headers.
  Risk Level: low, docs only
  Testing: n/a
  Docs Changes: updated
  Release Notes: n/a
diff: @@ -602,9 +602,13 @@ Supported variable names are:
  namespace and key(s) are specified as a JSON array of strings. Finally, percent symbols in the
  parameters **do not** need to be escaped by doubling them.
  + Upstream metadata cannot be added to request headers as the upstream host has not been selected
  + when custom request headers are generated.
  +
  %UPSTREAM_REMOTE_ADDRESS%
  Remote address of the upstream host. If the address is an IP address it includes both address
  - and port.
  + and port. The upstream remote address cannot be added to request headers as the upstream host
  + has not been selected when custom request headers are generated.
  %PER_REQUEST_STATE(reverse.dns.data.name)%
  Populates the header with values set on the stream info filterState() object. To be
  