doc(fpm_command_line): fix help text

Since version 0.8.0, fpm adds the `--flag` arguments to the default
arguments.  Overriding the defaults requires using the [fortran]
keyword in the manifest.  This commit edits the `--help` output to
reflect the current behavior.

src/fpm_command_line.f90

@@ -170,7 +170,8 @@ module fpm_command_line
     ' --flag  FFLAGS    selects compile arguments for the build, the default value is',&
     '                   set by the FPM_FFLAGS environment variable. These are added  ',&
     '                   to the profile options if --profile is specified, else these ',&
-    '                   options override the defaults. Note object and .mod          ',&
+    '                   are added to the defaults. To override the defaults, use the ',&
+    '                   keyword [fortran] in the manifest. Note object and .mod      ',&
     '                   directory locations are always built in.                     ',&
     ' --c-flag CFLAGS   selects compile arguments specific for C source in the build.',&
     '                   The default value is set by the FPM_CFLAGS environment       ',&