keto: Clarify glob matchin pattern (#225)

Closes ory/keto#141

docs/keto/engines/acp-ory.md

@@ -113,6 +113,37 @@ is:
   `cat` nor `bat`.
 - **alternatives list:** `{cat,bat,[mt]at}` matches `cat`, `bat`, `mat`, `tat`
   and nothing else.
+- **backslash:** `foo\\bar` matches `foo\bar` and nothing else. `foo\bar` matches
+ `foobar` and nothing else. `foo\*bar` matches `foo*bar` and nothing else. Please note
+ that when using JSON you need to double escape backslashes: `foo\\bar`
+ becomes `{"...": "foo\\\\bar"}`.
+
+The pattern syntax is:
+
+```
+  pattern:
+      { term }
+  
+  term:
+      `*`         matches any sequence of non-separator characters
+      `**`        matches any sequence of characters
+      `?`         matches any single non-separator character
+      `[` [ `!` ] { character-range } `]`
+                  character class (must be non-empty)
+      `{` pattern-list `}`
+                  pattern alternatives
+      c           matches character c (c != `*`, `**`, `?`, `\`, `[`, `{`, `}`)
+      `\` c       matches character c
+  
+  character-range:
+      c           matches character c (c != `\\`, `-`, `]`)
+      `\` c       matches character c
+      lo `-` hi   matches character c for lo <= c <= hi
+  
+  pattern-list:
+      pattern { `,` pattern }
+                  comma-separated (without spaces) patterns
+```
 
 ### Regular Expressions