Optimization options used to build the RTX libs included in the CMSIS-RTX pack are not documented

[chentang16]

The CMSIS-RTX documentation page e.g. https://arm-software.github.io/CMSIS-RTX/latest/cre_rtx_proj.html#creating_RTX5_LIB doesn't describe which optimization options used to build the RTX libs included in the CMSIS-RTX pack. After checking the RTX_Library.csolution.yml file shipped in the CMSIS-RTX pack, we see the option "optimize: size" is used there, which is corresponding to -Oz for Arm compiler v6.x or -Os for Clang/GCC. It is better documenting this detail in the manual page of CMSIS-RTX.