manual: complete example in error message documentation
The manual gives "an example showing how to handle failure to open a
file correctly." The example function, open_sesame, uses the
newly-introduced strerror function and errno and
program_invocation_short_name variables. It fails to specify GNU
extensions, however, so attempts to use it in the following way:
int main (void) {open_sesame ("badname");}
fail during compilation with "error: ‘program_invocation_short_name’
undeclared", indicating the example is incomplete. The presence of
"#include"s suggest everything neccesary for the function to work should
be present. For completeness, the example is lacking the following line:
#define _GNU_SOURCE
as the declarations of program_invocation_*name in errno.h are wrapped
in an "#ifdef __USE_GNU" conditional.
The documentation of the variables is also expanded, adding that their
definition lies in errno.h and noting specifically they are GNU
extensions.