doc: Enhancements for microkernel mailbox objects
doc: Enhancements for microkernel mailbox objects

Enhances description for mailbox definition. Adds examples of basic
mailbox use. Enhances descriptions in mailbox API table.

Note: Enhancement of the initial part of this document is left
for future commits, as are examples of more complex mailbox operations
like asynchronous send and receive.

Change-Id: I9a0b46f7e71242a113fab2ded395e068fefede84
Signed-off-by: Allan Stephens <1038bc33971d2c23c8e2eee8dd6938c077db52dc@windriver.com>

holtmann/zephyr,finikorg/zephyr,coldnew/zephyr-project-fork,galak/zephyr,bboozzoo/zephyr,jamesonwilliams/zephyr-kernel,fbsder/zephyr,jamesonwilliams/zephyr-kernel,coldnew/zephyr-project-fork,bboozzoo/zephyr,sharronliu/zephyr,mbolivar/zephyr,bboozzoo/zephyr,aceofall/zephyr-iotos,mirzak/zephyr-os,mbolivar/zephyr,punitvara/zephyr,runchip/zephyr-cc3220,holtmann/zephyr,pklazy/zephyr,aceofall/zephyr-iotos,zephyriot/zephyr,zephyrproject-rtos/zephyr,holtmann/zephyr,fractalclone/zephyr-riscv,nashif/zephyr,pklazy/zephyr,finikorg/zephyr,rsalveti/zephyr,bboozzoo/zephyr,bigdinotech/zephyr,jamesonwilliams/zephyr-kernel,erwango/zephyr,fbsder/zephyr,bigdinotech/zephyr,32bitmicro/zephyr,jamesonwilliams/zephyr-kernel,aceofall/zephyr-iotos,sharronliu/zephyr,fractalclone/zephyr-riscv,rsalveti/zephyr,explora26/zephyr,Vudentz/zephyr,zephyriot/zephyr,galak/zephyr,coldnew/zephyr-project-fork,punitvara/zephyr,tidyjiang8/zephyr-doc,mbolivar/zephyr,explora26/zephyr,aceofall/zephyr-iotos,explora26/zephyr,bigdinotech/zephyr,pklazy/zephyr,kraj/zephyr,bigdinotech/zephyr,erwango/zephyr,galak/zephyr,jamesonwilliams/zephyr-kernel,holtmann/zephyr,nashif/zephyr,rsalveti/zephyr,finikorg/zephyr,finikorg/zephyr,pklazy/zephyr,finikorg/zephyr,zephyriot/zephyr,runchip/zephyr-cc3200,aceofall/zephyr-iotos,explora26/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,fractalclone/zephyr-riscv,kraj/zephyr,galak/zephyr,Vudentz/zephyr,runchip/zephyr-cc3220,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,pklazy/zephyr,kraj/zephyr,bboozzoo/zephyr,mirzak/zephyr-os,erwango/zephyr,32bitmicro/zephyr,tidyjiang8/zephyr-doc,GiulianoFranchetto/zephyr,Vudentz/zephyr,kraj/zephyr,punitvara/zephyr,mbolivar/zephyr,rsalveti/zephyr,runchip/zephyr-cc3220,runchip/zephyr-cc3220,fractalclone/zephyr-riscv,galak/zephyr,mirzak/zephyr-os,GiulianoFranchetto/zephyr,tidyjiang8/zephyr-doc,punitvara/zephyr,runchip/zephyr-cc3200,Vudentz/zephyr,mirzak/zephyr-os,runchip/zephyr-cc3200,runchip/zephyr-cc3220,coldnew/zephyr-project-fork,sharronliu/zephyr,32bitmicro/zephyr,bigdinotech/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,rsalveti/zephyr,tidyjiang8/zephyr-doc,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,tidyjiang8/zephyr-doc,zephyrproject-rtos/zephyr,32bitmicro/zephyr,erwango/zephyr,sharronliu/zephyr,Vudentz/zephyr,zephyriot/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,sharronliu/zephyr,runchip/zephyr-cc3200,ldts/zephyr,mirzak/zephyr-os,fractalclone/zephyr-riscv,fbsder/zephyr,punitvara/zephyr,ldts/zephyr,Vudentz/zephyr,runchip/zephyr-cc3200,ldts/zephyr,coldnew/zephyr-project-fork,mbolivar/zephyr,erwango/zephyr,nashif/zephyr,32bitmicro/zephyr,zephyriot/zephyr,holtmann/zephyr,nashif/zephyr