doc: tweak wording for stdin detection

This makes it slightly more precise to cover weird cases like trying to
pass a directory on stdin.

Closes #2906

Author: BurntSushi

crates/core/flags/doc/template.rg.1

@@ -43,10 +43,10 @@ configuration file. The file can specify one shell argument per line. Lines
 starting with \fB#\fP are ignored. For more details, see \fBCONFIGURATION
 FILES\fP below.
 .sp
-ripgrep will automatically detect if stdin exists and search stdin for a regex
-pattern, e.g. \fBls | rg foo\fP. In some environments, stdin may exist when
-it shouldn't. To turn off stdin detection, one can explicitly specify the
-directory to search, e.g. \fBrg foo ./\fP.
+ripgrep will automatically detect if stdin is a readable file and search stdin
+for a regex pattern, e.g. \fBls | rg foo\fP. In some environments, stdin may
+exist when it shouldn't. To turn off stdin detection, one can explicitly
+specify the directory to search, e.g. \fBrg foo ./\fP.
 .sp
 Like other tools such as \fBls\fP, ripgrep will alter its output depending on
 whether stdout is connected to a tty. By default, when printing a tty, ripgrep