update docs/tutorials/building_r.md

modify the step for setting CFLAGS for a better explanation

Co-authored-by: Heather Turner <[email protected]>

Author: avinab-neogy

docs/tutorials/building_r.md

@@ -64,8 +64,7 @@ cd $BUILDDIR
 CFLAGS="-g -O0"
 ```
 
-- After running this command, the CFLAGS setting will apply only to the
-current terminal session.
+- These flags modify the configuration defined in the next step, so that when R is built, C code will be compiled with debugging symbols (`-g`) and compiler optimizations will be disabled (`-O0`) so that the structure of the code closely matches the original source.
 
 **6) Configure the build**