[doc][index] doxygen 1.8.4 complains that SYMBOL_CACHE_SIZE is obsolete
and suggests to remove the entry
[doc] add algorithms/detail/comparable_distance to the doxygen input
path
[doc] add documentation support for is_valid and is_simple
Merge branch 'feature/doc' into feature/is_simple
Merge pull request #76 from mkaravel/feature/doc

Fix documentation issues
Merge branch 'develop' of https://github.com/boostorg/geometry into feature/is_simple
[algorithms][is_simple] add missing include
[geometry] add is_simple and is_valid algorithms in geometry.hpp
[doc][is_simple, is_valid] add doxygen and qbk support for documentation
of is_simple and is_valid; add qbk doc for is_simple and is_valid;
add example for is_simple and is_valid;
[algorithms][is_simple,is_valid] add doxygen documentation for is_simple and is_valid