parser: Document that XML_PARSE_NOBLANKS is broken

Long text content can generate multiple "characters" callbacks which can
lead to NOBLANKS removing whitespace in non-whitespace text nodes. So
the NOBLANKS option doesn't even work reliably with the pull parser.
This would be extremely hard to fix.

Unfortunately, `xmllint --format` relies on this option which is another
reason why this feature never really worked.

doc/xmllint.xml

@@ -283,6 +283,10 @@
                     environment variable controls the indentation. The default value is two
                     spaces "  ").
                 </para>
+                <para>
+                    Especially in the absence of a DTD, this feature has never worked reliably
+                    and is fundamentally broken.
+                </para>
             </listitem>
         </varlistentry>