updated info on lang-specific subdirs in Doc/Manual/Library.html

Hi,

the documentation chapter 8.1

http://swig.org/Doc2.0/SWIGDocumentation.html#Library_nn2 

states that for each search directory, also language-specific subdirectory is searched for interface files. This is not true. In fact, this applies only to "library" paths (system-wide and local ./swig_lib") and does not apply to local directory and directories defined with -I option. This could be potentially fixed by modifying "Source/Module/main.cxx", but it could introduce more troubles that advantages (think about those, who do not want lang-specific subdirs for -I and local dirs), so I propose this small change to the documentation.