src/MANIFEST.in: Exclude generated file farey_symbol.h; add comments

.gitignore

@@ -131,9 +131,15 @@ __pycache__/
 # Generated Cython files
 *.so
 **/*.so
+/src/cython_debug
+# Most C and C++ files are generated by Cython and should not
+# be included in the sdist.
 /src/sage/**/*.c
 /src/sage/**/*.cpp
+# C header generated by Cython
 /src/sage/modular/arithgroup/farey_symbol.h
+# List of C and C++ files that are actual source files,
+# NOT generated by Cython. The same list appears in src/MANIFEST.in
 !/src/sage/cpython/debugimpl.c
 !/src/sage/graphs/base/boost_interface.cpp
 !/src/sage/graphs/cliquer/cl.c
@@ -156,7 +162,6 @@ __pycache__/
 !/src/sage/stats/distributions/dgs_gauss_dp.c
 !/src/sage/stats/distributions/dgs_gauss_mp.c
 !/src/sage/symbolic/ginac/*.cpp
-/src/cython_debug
 
 # Temporary build files
 build/temp.*/

src/MANIFEST.in

@@ -3,9 +3,17 @@ prune sage_setup
 prune sage_docbuild
 prune doc
 
+#
+# Most C and C++ files are generated by Cython and should not
+# be included in the sdist.
+#
 global-exclude *.c
 global-exclude *.cpp
-include sage/modular/arithgroup/farey_symbol.h
+
+#
+# List of C and C++ files that are actual source files,
+# NOT generated by Cython. The same list appears in SAGE_ROOT/.gitignore
+#
 include sage/cpython/debugimpl.c
 include sage/graphs/base/boost_interface.cpp
 include sage/graphs/cliquer/cl.c