bpo-33000: Document that IDLE's shell has no line limit. (#10373)
A program that runs indefinitely can overfill memory.
diff --git a/Lib/idlelib/help.html b/Lib/idlelib/help.html
index 83bd4a1..f0b4215 100644
--- a/Lib/idlelib/help.html
+++ b/Lib/idlelib/help.html
@@ -648,14 +648,21 @@
corresponding output device. When IDLE executes user code, <code class="docutils literal notranslate"><span class="pre">sys.stdout</span></code>
and <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code> are connected to the display area of IDLE’s Shell. Some of
its features are inherited from the underlying Tk Text widget. Others
-are programmed additions.</p>
+are programmed additions. Where it matters, Shell is designed for development
+rather than production runs.</p>
+<p>For instance, Shell never throws away output. A program that sends unlimited
+output to Shell will eventually fill memory, resulting in a memory error.
+In contrast, some system text windows only keep the last n lines of output.
+A Windows console, for instance, keeps a user-settable 1 to 9999 lines,
+with 300 the default.</p>
<p>Text widgets display a subset of Unicode, the Basic Multilingual Plane (BMP).
Which characters get a proper glyph instead of a replacement box depends on
the operating system and installed fonts. Newline characters cause following
-text to appear on a new line, but other control characters are replaced
-with a box. But note that the <code class="docutils literal notranslate"><span class="pre">repr()</span></code> function, which is used for
-interactive echo of expression values, replaces control characters
-with escape codes before they are output.</p>
+text to appear on a new line, but other control characters are either
+replaced with a box or deleted. However, <code class="docutils literal notranslate"><span class="pre">repr()</span></code>, which is used for
+interactive echo of expression values, replaces control characters,
+some BMP codepoints, and all non-BMP characters with escape codes
+before they are output.</p>
<p>Normal and error output are generally kept separate (on separate lines)
from code input and each other. They each get different highlight colors.</p>
<p>For SyntaxError tracebacks, the normal ‘^’ marking where the error was