doc: add notice about nginx location directive when shaarli is installed in a subdirectory

- fixes https://github.com/shaarli/Shaarli/issues/1679

doc/md/Reverse-proxy.md

@@ -118,6 +118,7 @@ http {
         ssl_certificate       /path/to/certificate
         ssl_certificate_key   /path/to/private/key
 
+        # if shaarli is installed in a subdirectory of the main domain, edit the location accordingly
         location / {
             proxy_set_header  X-Real-IP         $remote_addr;
             proxy_set_header  X-Forwarded-For   $proxy_add_x_forwarded_for;

doc/md/Server-configuration.md

@@ -304,6 +304,7 @@ server {
     client_max_body_size 100m;
 
     # relative path to shaarli from the root of the webserver
+    # if shaarli is installed in a subdirectory of the main domain, edit the location accordingly
     location / {
         # default index file when no file URI is requested
         index index.php;