doc: Initial documentation of pseudomodules

Co-authored-by: Francisco <femolina@uc.cl>

doc/doxygen/riot.doxyfile

@@ -275,7 +275,9 @@ OPTIMIZE_OUTPUT_VHDL   = NO
 # Note that for custom extensions you also need to set FILE_PATTERNS otherwise
 # the files are not read by doxygen.
 
-EXTENSION_MAPPING      =
+# Python is close enough that we can have Makefile comments starting with `##`
+# that are both recognized by Doxygen and comments to Make
+EXTENSION_MAPPING      = mk=Python
 
 # If the MARKDOWN_SUPPORT tag is enabled then doxygen pre-processes all comments
 # according to the Markdown format, which allows for more readable
@@ -771,7 +773,8 @@ INPUT                  = ../../doc.txt \
                          src/emulators.md \
                          src/release-cycle.md \
                          src/changelog.md \
-                         ../../LOSTANDFOUND.md
+                         ../../LOSTANDFOUND.md \
+                         ../../makefiles/pseudomodules.inc.mk
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses

makefiles/pseudomodules.inc.mk

@@ -1,3 +1,21 @@
+## @defgroup pseudomodules Pseudomodules
+## @brief Modules without dedicated files
+##
+## These are implemented in other modules or core components,
+## and serve to enable certain functionality.
+##
+## Pseudomodules are used instead of plain defines (that would be set using `CFLAGS += -DMODULE_NAME`)
+## because they can participate in dependency resolution:
+## they can pull in other modules.
+##
+## Pseudomodules are often enabled automatically through module dependencies,
+## but can also be enabled manually by stating `USEMODULE += module_name` in the Makefile.
+##
+## The list of documented pseudomodules is not comprehensive by far;
+## @ref makefiles/pseudomodules.inc.mk lists all that are not defined inside their main modules.
+##
+## @{
+
 PSEUDOMODULES += atomic_utils
 PSEUDOMODULES += base64url
 PSEUDOMODULES += board_software_reset
@@ -214,3 +232,5 @@ NO_PSEUDOMODULES += auto_init_usbus
 NO_PSEUDOMODULES += auto_init_screen
 
 # Packages and drivers may also add modules to PSEUDOMODULES in their `Makefile.include`.
+
+## @}