improve manpage; show data-dir in -v

pdf2htmlEX.1.in

@@ -208,7 +208,7 @@ Whether to process non-text objects (as images)
 
 .TP
 .B --data-dir <dir> (Default: @CMAKE_INSTALL_PREFIX@/share/pdf2htmlEX)
-Specify the folder holding the manifest and other files
+Specify the folder holding the manifest and other files (see below for the manifest file)`
 
 .TP
 .B --css-draw <0|1> (Default: 0)
@@ -231,6 +231,15 @@ Print copyright and version info
 Print usage information
 
 
+.SH MANIFEST and DATA-DIR
+When split-pages is 0, the manifest file describes how the final html page should be generated.
+
+By default, pdf2htmlEX will use the manifest in the default data-dir (run `pdf2htmlEX -v` to check), which gives a simple demo of its syntax.
+
+You can modify the default one, or you can create a new one and specify the correct data-dir in the command line. 
+
+When single-html is 1, all files referred by the manifest must be located in the data-dir.
+
 .SH EXAMPLE
 .TP
 .B pdf2htmlEX /path/to/file.pdf

src/pdf2htmlEX.cc

@@ -48,6 +48,7 @@ void show_version_and_exit(const char * dummy = nullptr)
     cerr << "Libraries: ";
     cerr << "poppler " << POPPLER_VERSION << ", ";
     cerr << "libfontforge " << ffw_get_version() << endl;
+    cerr << "Default data-dir: " << PDF2HTMLEX_DATA_PATH << endl;
     exit(EXIT_SUCCESS);
 }