improve documentation

This partly reverts c62a44195c82d02f87d5e18db6f86bf191e2c142.
Improve read()
Improve s:read()
fix documentation

vimproc#version() and vimproc#dll_version() return a Number, not a
String.
Merge pull request #145 from k-takata/topic/fix-doc-vimproc-version

fix documentation of vimproc#version() and vimproc#dll_version()
Improve read join
number parameter didn't work

(after bea18e2771455ec164f8d951dc2fc48987db073c)
fix number parameter