DOC: configuration: wrap long line for "strstr()" conditional expression

This keyword had too long a description line, let's split it. This can be
backported to 2.8.

doc/configuration.txt

@@ -1118,7 +1118,8 @@ The list of currently supported predicates is the following:
 
   - streq(<str1>,<str2>)  : returns true only if the two strings are equal
   - strneq(<str1>,<str2>) : returns true only if the two strings differ
-  - strstr(<str1>,<str2>) : returns true only if the second string is found in the first one
+  - strstr(<str1>,<str2>) : returns true only if the second string is found in
+                            the first one.
 
   - version_atleast(<ver>): returns true if the current haproxy version is
                             at least as recent as <ver> otherwise false. The