📝 fixed Doxygen warnings

doc/Doxyfile

-# Doxyfile 1.8.14
+# Doxyfile 1.8.15
 
 #---------------------------------------------------------------------------
 # Project related configuration options
@@ -12,6 +12,7 @@ OUTPUT_DIRECTORY       = .
 CREATE_SUBDIRS         = NO
 ALLOW_UNICODE_NAMES    = NO
 OUTPUT_LANGUAGE        = English
+OUTPUT_TEXT_DIRECTION  = None
 BRIEF_MEMBER_DESC      = YES
 REPEAT_BRIEF           = NO
 ABBREVIATE_BRIEF       =
@@ -27,11 +28,11 @@ MULTILINE_CPP_IS_BRIEF = NO
 INHERIT_DOCS           = YES
 SEPARATE_MEMBER_PAGES  = YES
 TAB_SIZE               = 4
-ALIASES                = "complexity=@par Complexity\n" \
-                         "liveexample{2}=@par Example\n \1 \n @includelineno \2.cpp \n Output (play with this example @htmlinclude \2.link):\n @verbinclude \2.output \n The <a href= https://github.com/nlohmann/json/blob/develop/doc/examples/\2.cpp>example code</a> above can be translated with @verbatim g++ -std=c++11 -Isrc doc/examples/\2.cpp -o \2 @endverbatim" \
-                         "requirement=@par Requirements\n" \
-                         "exceptionsafety=@par Exception safety\n" \
-                         "iterators=@par Iterator validity\n"
+ALIASES                = "complexity=@par Complexity^^" \
+                         "liveexample{2}=@par Example^^ \1 ^^ @includelineno \2.cpp \n Output (play with this example @htmlinclude \2.link):^^ @verbinclude \2.output ^^ The <a href= https://github.com/nlohmann/json/blob/develop/doc/examples/\2.cpp>example code</a> above can be translated with @verbatim g++ -std=c++11 -Isrc doc/examples/\2.cpp -o \2 @endverbatim" \
+                         "requirement=@par Requirements^^" \
+                         "exceptionsafety=@par Exception safety^^" \
+                         "iterators=@par Iterator validity^^"
 TCL_SUBST              =
 OPTIMIZE_OUTPUT_FOR_C  = NO
 OPTIMIZE_OUTPUT_JAVA   = NO
@@ -107,7 +108,8 @@ WARN_LOGFILE           =
 # Configuration options related to the input files
 #---------------------------------------------------------------------------
 INPUT                  = ../src/json.hpp \
-                         index.md faq.md
+                         index.md \
+                         faq.md
 INPUT_ENCODING         = UTF-8
 FILE_PATTERNS          =
 RECURSIVE              = NO
@@ -138,6 +140,7 @@ USE_HTAGS              = NO
 VERBATIM_HEADERS       = NO
 CLANG_ASSISTED_PARSING = YES
 CLANG_OPTIONS          = -std=c++11
+CLANG_COMPILATION_DATABASE_PATH                                        = 0
 #---------------------------------------------------------------------------
 # Configuration options related to the alphabetical class index
 #---------------------------------------------------------------------------