Documentation improvements plus Ethernet turn-off fix

[alrvid]

1. Improve Standby Mode example by adding how to go into Standby Mode waiting for only a wakeup pin.

2. Fix two incorrect comments.

3. Add option to not turn off Ethernet. This option is added for libraries that turn off the rail that powers
   the Ethernet chip, and it prevents malfunction because of the combination of libraries.

4. Improve template function Doxygen documentation. Adds one declaration for Doxygen and another for the compiler.
   Plus improves a bit on the documentation.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ alrvid
❌ actions-user
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions]

Memory usage change @ c4cce7a

Board	flash	%	RAM for global variables	%
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A

Click for full report table

Board	examples/AllowDeepSleep flash	%	examples/AllowDeepSleep RAM for global variables	%	examples/DeepSleepLockDebug flash	%	examples/DeepSleepLockDebug RAM for global variables	%	examples/Standby flash	%	examples/Standby RAM for global variables	%
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A

Click for full report CSV

Board,examples/AllowDeepSleep<br>flash,%,examples/AllowDeepSleep<br>RAM for global variables,%,examples/DeepSleepLockDebug<br>flash,%,examples/DeepSleepLockDebug<br>RAM for global variables,%,examples/Standby<br>flash,%,examples/Standby<br>RAM for global variables,%
arduino:mbed_portenta:envie_m7,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A

[sebromero]

Let's use DOXYGEN_ONLY as the define so that it starts with DOXYGEN and is easier to spot.

[sebromero]

LGTM