Explain expected usage of log body (#2096)

Resolves #2066 and #1752

Supports #2068

## Changes

Adds a note to the log data model which explains the intended usage of the `Body` field. 

## Additional Context

Extensive discussion has been had on this issue on [#1613](#1613 (comment)), as well as in the Log SIG group.

specification/logs/data-model.md

@@ -398,8 +398,10 @@ Type: any.
 Description: A value containing the body of the log record (see the description
 of `any` type above). Can be for example a human-readable string message
 (including multi-line) describing the event in a free form or it can be a
-structured data composed of arrays and maps of other values. Can vary for each
-occurrence of the event coming from the same source. This field is optional.
+structured data composed of arrays and maps of other values. First-party
+Applications SHOULD use a string message. However, a structured body may be
+necessary to preserve the semantics of some existing log formats. Can vary for
+each occurrence of the event coming from the same source. This field is optional.
 
 ### Field: `Resource`