libc: add man page for uexterr_gettext(3)

[mckusick]

Fix some grammar nits.

lib/libc/gen/uexterr_gettext.3

32

which returned an extended error

37

should use the

41

with an advanced

43

with the extended error

46

the kernel

54

function returns

55

There are

58

would be => is

69

is a

[kib]

Fix grammar.