apache frontend: describe prefer-gzip-encoding-to-backend

From Cédric message

When a client do a request, you pass the Accept-Encoding header. For me it is:
```
Accept-Encoding:gzip, deflate, sdch, br
```

But in a lot of case the cache configuration has `Vary:Accept-Encoding` . `Accept-Encoding` request header may vary a lot from a client to another but one of the common denominator is `gzip`. So in order to avoid a multiplication of cache versions in CDN, if `gzip` is present in the `Accept-Encoding` request header all other format are stripped from the request in order to reduce variation in Trafficserver.

When this parameter is activated:
```
Accept-Encoding:gzip, deflate, sdch, br
```
is transmitted as
```
Accept-Encoding:gzip
```
And
```
Accept-Encoding:deflate, sdch, br
```
is transmitted as
```
Accept-Encoding:deflate, sdch, br
```

software/apache-frontend/instance-slave-apache-input-schema.json

@@ -117,7 +117,7 @@
 
     "prefer-gzip-encoding-to-backend": {
       "title": "Prefer gzip Encoding for Backend",
-      "description": "If set to true, if a request is made with accept encoding 'gzip', only that one will be transferred to the backend",
+      "description": "If set to true, frontend will rewrite Accept-Encoding request header to simply 'gzip' for all variants of Accept-Encoding containing 'gzip', in order to maximize cache hits for resources cached with Vary: Accept-Encoding when enable_cache is used",
       "type": "string",
       "default": "false",
       "enum": ["false", "true"]