Add missing value placeholders to usage message

src/bin/sfdoc

             "-d, --debug        Turn on debugging"                      \
             "-a, --all          Don't ignore hidden (starting with"     \
             "                   underscore) modules or objects"         \
-            "--encoding         Override encoding of the source text"   \
+            "--encoding ENC     Override encoding of the source text"   \
             "                   (default: utf8)"                        \
-            "--name             Override module name (useful when"      \
+            "--name NAME        Override module name (useful when"      \
             "                   exporting from a file and the filename" \
             "                   is not helpful)"                        \
             "-I PTH, --include PTH  Include path PTH when searching"    \