docs: fix serializer javadocs to use 'writing' instead of 'reading'

[Copilot]

Summary

Fixed incorrect terminology in the serializer.hpp documentation. The javadocs were using "reading" terminology when they should use "writing" to correctly describe the serializer's purpose.

Background

The serializer is used to write HTTP messages to remote peers (e.g., with asio::write), while the parser is used to read from remote peers (e.g., with asio::read). The documentation should consistently reflect this perspective.

Changes

Updated the start_stream() method documentation in three places:

1. Main description (line 340-341): Changed from "for reading the body from an external source" to "for writing the body to an external destination"

2. Control flow description (line 349): Changed from "while reading from an external source" to "for writing to the remote peer"

3. Return value description (line 400): Changed from "for reading body content into" to "for writing body content into"

Example

The corrected documentation now properly describes the serializer's role:

// Returns a stream object for WRITING the body to an external destination
serializer::stream strm = serializer.start_stream(response);

This aligns with typical usage where the serializer is used with write operations:

write_some(client, serializer);  // Writing to the remote peer

Fixes issue where the javadocs incorrectly suggested the serializer was for reading data instead of writing it.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>docs: serializer javadocs are backwards</issue_title>
<issue_description>For example here:
https://github.com/cppalliance/http_proto/blob/develop/include/boost/http_proto/serializer.hpp#L340

        Initializes the serializer with the HTTP
        start-line and headers from `m`, and returns
        a @ref stream object for reading the body
        from an external source.

This should say "writing the body" because we always word things from the perspective of the remote peer. That is, we write the body (to the remote peer). We read the body (from the remote peer). Serializer is used with asio::write for example. We don't use asio::read, as that is for parser.</issue_description>

<agent_instructions>Do not change anything except comments in the file "include/boost/http_proto/serializer.hpp"</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #176

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.48%. Comparing base (1a8a81c) to head (ef5a587).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #177   +/-   ##
========================================
  Coverage    92.48%   92.48%           
========================================
  Files           71       71           
  Lines         4854     4854           
========================================
  Hits          4489     4489           
  Misses         365      365           

Files with missing lines	Coverage Δ
include/boost/http_proto/serializer.hpp	100.00% <ø> (ø)

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