add man pages for array API

array/array_length.3

@@ -0,0 +1,20 @@
+.TH array_length 3
+.SH NAME
+array_length \- get number of allocated members in array
+.SH SYNTAX
+.B #include <array.h>
+
+int64 \fBarray_length\fP(array* \fIx\fR,uint64 \fImembersize\fR);
+
+  array \fIx\fR;
+  int64 members = array_length(&\fIx\fR,sizeof(\fIt\fR));
+
+.SH DESCRIPTION
+array_length returns the number of initialized bytes in \fIx\fR, divided
+by the size of \fIt\fR. In other words, array_get will succeed for
+positions 0 through array_length-1; it will fail for position
+array_length.
+
+If \fIx\fR is unallocated, array_length and array_bytes return 0.
+.SH "SEE ALSO"
+array_allocate(3), array_get(3), array_fail(3), array_bytes(3)