doc: expect sed in /usr/bin. env is no good use.

doc/ref/tags/.generate

@@ -1,11 +1,16 @@
 #!/bin/bash
 
+if [ ! -x /usr/bin/sed ]; then
+    echo "sed not found in /usr/bin, please consider adding a symlink to it there." &>2
+    exit 1
+fi
+
 scanner=$ZOTONIC_SRC/src/erlydtl/erlydtl_scanner.erl
 dir=${0%/*}
 
 for tag in `$dir/.get_tags $scanner`
 do
-    if grep $tag $dir/.ignore >/dev/null; then
+    if grep "$tag" $dir/.ignore >/dev/null; then
         continue
     fi
 

doc/ref/tags/.get_tags

@@ -1,4 +1,4 @@
-#!/usr/bin/env sed -f
+#!/usr/bin/sed -f
 
 # operate on the first lines of the identifier_to_keyword fun
 # that mentions open_tag (guard clause), and down until the

doc/ref/tags/.ignore

@@ -1,2 +1,3 @@
 else
 elseif
+elif