examples: added descriptions

docs/examples/multi-app.c

docs/examples/postinmemory.c

docs/examples/postit2.c

docs/examples/progressfunc.c

docs/examples/resolve.c

docs/examples/rtsp.c

docs/examples/sampleconv.c

docs/examples/sendrecv.c

docs/examples/sepheaders.c

docs/examples/sessioninfo.c

docs/examples/sftpget.c

docs/examples/simplepost.c

docs/examples/simplessl.c

docs/examples/smooth-gtk-thread.c

docs/examples/synctime.c

docs/examples/threaded-ssl.c

docs/examples/url2file.c

docs/examples/usercertinmem.c

docs/examples/xmlstream.c