add buffer_fromsa (make virtual buffer from stralloc)

buffer/buffer_fromsa.3

@@ -0,0 +1,19 @@
+.TH buffer_fromsa 3
+.SH NAME
+buffer_fromsa \- initialize buffer structure from stralloc
+.SH SYNTAX
+.B #include <buffer.h>
+
+int \fBbuffer_fromsa\fR(buffer* \fIb\fR,stralloc* \fIsa\fR);
+.SH DESCRIPTION
+buffer_fromsa makes a virtual read buffer from a stralloc.  The buffer
+reading functions will be able to read until the end of the data in the
+stralloc, then signal EOF.  The buffer does not have its own buffer
+space, it reuses the one in the stralloc.
+
+Do not touch the stralloc until your are done with the buffer!  In
+particular, do not call stralloc_free on it!
+
+Reading data from the buffer will not change the stralloc.
+.SH "SEE ALSO"
+buffer_init(3), buffer(3)