doc: note ripgrep's stdin behavior

Fixes #1439

CHANGELOG.md

@@ -55,6 +55,8 @@ Bug fixes:
   Document usage of `--type all`.
 * [BUG #1389](https://github.com/BurntSushi/ripgrep/issues/1389):
   Fixes a bug where ripgrep would panic when searching a symlinked directory.
+* [BUG #1439](https://github.com/BurntSushi/ripgrep/issues/1439):
+  Improve documentation for ripgrep's automatic stdin detection.
 * [BUG #1445](https://github.com/BurntSushi/ripgrep/issues/1445):
   ripgrep now respects ignore rules from .git/info/exclude in worktrees.
 * [BUG #1485](https://github.com/BurntSushi/ripgrep/issues/1485):

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'.
 

doc/rg.1.txt.tpl

@@ -41,6 +41,11 @@ 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*.