Directions for generating documentation are wrong

The directions for generating the documentation say

> The documentation of the functions of OpENer is part of the source code. The source packages contain the generated documentation in the directory doc/api_doc. If you use the GIT version you will need the program Doxygen for generating the HTML documentation. You can generate the documentation by invoking doxygen from the command line in the opener main directory.

This doesn't work. It results in:

```
warning: Tag 'TCL_SUBST' at line 237 of file 'source/opener.doxyfile.in' has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
warning: Tag 'COLS_IN_ALPHA_INDEX' at line 1020 of file 'source/opener.doxyfile.in' has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
warning: Tag 'PERL_PATH' at line 2073 of file 'source/opener.doxyfile.in' has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
warning: Tag 'MSCGEN_PATH' at line 2095 of file 'source/opener.doxyfile.in' has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
warning: argument 'a4wide' for option PAPER_TYPE is not a valid enum value
Using the default: a4!
warning: tag INPUT: input source '@OpENer_SOURCE_DIR@/src' does not exist
warning: tag INPUT: input source '@PROJECT_BINARY_DIR@/src/ports/devicedata.h' does not exist
Doxygen version used: 1.9.1
error: tag OUTPUT_DIRECTORY: Output directory '@OpENer_BINARY_DIR@/api_doc' does not exist and cannot be created
Exiting...

```