gh-143774 - Improve IDLE Format Paragraph doc (GH-143775)

Add a reminder to not rewrap code line to the Menu => Format => Reformat Paragraph entry.
In Editing and Nagivagion, add a new 'Format block' subsection that defines 'paragraph'
to better match what is dependably handled as more or less expected.
In particular, specify equal indents and that the resulting indent equals original indent.
Also mention that selections are expanded to complete lines and how to modify max length.

(Also fix a couple case errors in cross references.)
(cherry picked from commit fa3abf5)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Author: terryjreedy

Doc/library/idle.rst

@@ -158,7 +158,7 @@ Go to Line
 
 Show Completions
    Open a scrollable list allowing selection of existing names. See
-   :ref:`Completions <completions>` in the Editing and navigation section below.
+   :ref:`Completions <completions>` in the Editing and Navigation section below.
 
 Expand Word
    Expand a prefix you have typed to match a full word in the same window;
@@ -167,7 +167,7 @@ Expand Word
 Show Call Tip
    After an unclosed parenthesis for a function, open a small window with
    function parameter hints.  See :ref:`Calltips <calltips>` in the
-   Editing and navigation section below.
+   Editing and Navigation section below.
 
 Show Surrounding Parens
    Highlight the surrounding parenthesis.
@@ -178,9 +178,9 @@ Format menu (Editor window only)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Format Paragraph
-   Reformat the current blank-line-delimited paragraph in comment block or
-   multiline string or selected line in a string.  All lines in the
-   paragraph will be formatted to less than N columns, where N defaults to 72.
+   Rewrap the text block containing the text insert cursor.
+   Avoid code lines.  See :ref:`Format block<format-block>` in the
+   Editing and Navigation section below.
 
 Indent Region
    Shift selected lines right by the indent width (default 4 spaces).
@@ -566,6 +566,20 @@ In an editor, import statements have no effect until one runs the file.
 One might want to run a file after writing import statements, after
 adding function definitions, or after opening an existing file.
 
+.. _format-block:
+
+Format block
+^^^^^^^^^^^^
+
+Reformat Paragraph rewraps a block ('paragraph') of contiguous equally
+indented non-blank comments, a similar block of text within a multiline
+string, or a selected subset of either.
+If needed, add a blank line to separate string from code.
+Partial lines in a selection expand to complete lines.
+The resulting lines have the same indent as before
+but have maximum total length of N columns (characters).
+Change the default N of 72 on the Window tab of IDLE Settings.
+
 .. _code-context:
 
 Code Context

Lib/idlelib/help.html

@@ -0,0 +1 @@
+Better explain the operation of Format / Format Paragraph.