Add SFDOC_HIDE_REGEX to enable overriding definition of "hidden"

src/include-bash/sfdoc.sh

 # with `_` (underscore).  These are ignored by default.  Set this to 'true' to show
 # them.
 #
+# See also $SFDOC_HIDE_REGEX.
+#
 SFDOC_SHOW_HIDDEN=${SFDOC_SHOW_HIDDEN:-false}
 
+#
+# Regex to override what is hidden
+#
+# Override definition of which objects are considered "private", thus hidden
+# by default.  Regular expression is used as Extended Regular Expressions,
+# is anchored on both sides and case-sensitive, e setting 'foo|bAr' matches
+# only objects whose entire name is either 'foo' or 'bAr'.
+#
+# See also $SFDOC_SHOW_HIDDEN.
+#
+SFDOC_HIDE_REGEX=${SFDOC_HIDE_REGEX:-_.*}
+
 sfdoc__export() {
     #
     # Export docs of module $2 from file $3 as format $3
     #
     local what=$1
     $SFDOC_SHOW_HIDDEN && cat && return
+    debug -v SFDOC_HIDE_REGEX
     case $what in
-        m)  grep -v ^_ ;;
-        v)  grep -v :_ ;;
-        f)  grep -v :_ ;;
+        m)  grep -vE "^$SFDOC_HIDE_REGEX$" ;;
+        v)  grep -vE ":$SFDOC_HIDE_REGEX$" ;;
+        f)  grep -vE ":$SFDOC_HIDE_REGEX$" ;;
         *)  warn "bug: invalid object type: $otype" ;;
     esac
 }

tests/shellfu_api/oracle/variables.stdout

 pretty:PRETTY_DEBUG_EXCLUDE
 pretty:PRETTY_USAGE
 pretty:PRETTY_VERBOSE
+sfdoc:SFDOC_HIDE_REGEX
 sfdoc:SFDOC_SHOW_HIDDEN
 sfpi:SFPI__PREFIX
 termcolors:TERMCOLORS_BLACK