doc: note ripgrep's stdin behavior

Fixes #1439

crates/core/app.rs

@@ -27,6 +27,11 @@ configuration file. The file can specify one shell argument per line. Lines
 starting with '#' are ignored. For more details, see the man page or the
 README.
 
+ripgrep will automatically detect if stdin exists and search stdin for a regex
+pattern, e.g. 'ls | rg foo'. In some environments, stdin may exist when it
+shouldn't. To turn off stdin detection explicitly specify the directory to
+search, e.g. 'rg foo ./'.
+
 Tip: to disable all smart filtering and make ripgrep behave a bit more like
 classical grep, use 'rg -uuu'.