Elaborate documentation of file-related system calls

[mortbopet]

As described in #121, the system call menu help->system calls has lacking documentation for the usage and meaning of file-related system calls, such as open.

These descriptions should be elaborated, such as by including the semantics of the various flags for open:

Ripes/src/syscall/systemio.h

Lines 80 to 86 in d692d71

	static constexpr int O_RDONLY = 0x00000000;
	static constexpr int O_WRONLY = 0x00000001;
	static constexpr int O_RDWR = 0x00000002;
	static constexpr int O_APPEND = 0x00000008;
	static constexpr int O_CREAT = 0x00000200; // 512
	static constexpr int O_TRUNC = 0x00000400; // 1024
	static constexpr int O_EXCL = 0x00000800; // 2048

i.e.

"Opens a file from a path" is pretty non-informative, and could easily also explain something about the flags.

[Satarupa22-SD]

@mortbopet I have added a PR on this issue. Could you please check and let me know if any additional changes are to be made. Thanks!