CC_SHELL_ESCAPED_FLAGS: improve docs

[xbjfk]

Hello, after a little thinking I realise I wasn't too happy with the docs I wrote and think they are verbose - and decided to make it more concise and in plain English:

• CC_SHELL_ESCAPED_FLAGS - if set, *FLAGS will be parsed as if they were shell arguments (similar to make and cmake) rather than splitting them on each space. For example, with CFLAGS='a "b c"', the compiler will be invoked with 2 arguments - a and b c - rather than 3: a, "b and c".

Thanks :)

[NobodyXu]

Thanks, the new doc is simpler to read.