Document casing for CMake build types

!5411 (merged) clarifies that build types are handled case insensitive. The wording is confusing

Note that configuration names are case-insensitive. The value of this variable will be the same as it is specified when invoking CMake. For instance, if -DCMAKE_BUILD_TYPE=ReLeAsE is specified, then the value of CMAKE_BUILD_TYPE will be ReLeAsE.

• if the configuration is case insensitive, explaining the case preserving nature is not necessary
• a recent discussion in 3990 showed some reader assume that CMake is self-aware of this constraint, i. e. instead of handling case sensitivity by themselfes:

string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
if (NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "ANYCASEWILLDO")
  set(CMAKE_BUILD_TYPE anYcASEwilldo)
endif()

they falsely assume CMake to handle it

if(${CMAKE_BUILD_TYPE} STREQUAL "anYcASEwilldo")
    set(CMAKE_BUILD_TYPE anYcASEwilldo)
endif()

i assume this is due to increased context awareness of recent scripting languages.

• clarify how build types are handled, i.e. whether Release is the same or different to release
  • projects can only be consumed via add subdirectory when build type conventions are compatible
  • the CMake project itself may use a convention already, which should indicate which option to recommend
    • otherwise, the CMake project may not need to take a position on how to handle build types

i'll start looking into this issue.