DOC: mention that "compression offload" is ignored in defaults section

This one is not inherited from defaults into frontends nor backends
because it would create a confusion situation where it would be hard
to disable it (since both frontend and backend would enable it).
(cherry picked from commit ffea9fde3848badb0c17ee7ff68069e95c7764b1)
[RELEASE] Released version 1.5.2

Released version 1.5.2 with the following main changes :
    - BUG/MEDIUM: backend: Update hash to use unsigned int throughout
    - BUG/MINOR: ssl: Fix external function in order not to return a pointer on an internal trash buffer.
    - DOC: expand the docs for the provided stats.
    - BUG/MEDIUM: unix: do not unlink() abstract namespace sockets upon failure.
    - MINOR: stats: fix minor typo in HTML page
    - BUG/MEDIUM: http: fetch "base" is not compatible with set-header
    - BUG/MINOR: counters: do not untrack counters before logging
    - BUG/MAJOR: sample: correctly reinitialize sample fetch context before calling sample_process()
    - MINOR: stick-table: make stktable_fetch_key() indicate why it failed
    - BUG/MEDIUM: counters: fix track-sc* to wait on unstable contents
    - BUILD: remove TODO from the spec file and add README
    - MINOR: log: make MAX_SYSLOG_LEN overridable at build time
    - MEDIUM: log: support a user-configurable max log line length
    - DOC: provide an example of how to use ssl_c_sha1
    - BUILD: http: fix isdigit & isspace warnings on Solaris
    - BUG/MINOR: listener: set the listener's fd to -1 after deletion
    - BUG/MEDIUM: unix: failed abstract socket binding is retryable
    - MEDIUM: listener: implement a per-protocol pause() function
    - MEDIUM: listener: support rebinding during resume()
    - BUG/MEDIUM: unix: completely unbind abstract sockets during a pause()
    - DOC: explicitly mention the limits of abstract namespace sockets
    - DOC: minor fix on {sc,src}_kbytes_{in,out}
    - DOC: fix alphabetical sort of converters
    - BUG/MAJOR: http: correctly rewind the request body after start of forwarding
    - DOC: remove references to CPU=native in the README
    - DOC: mention that "compression offload" is ignored in defaults section