manuals: replace hyphens with em dashes

Fix some hyphens being improperly used as em dashes.
See https://www.grammarly.com/blog/hyphens-and-dashes/

Using em dashes may also allow Sphinx to hyphenate
and break lines in the best way.

Note that the first character after an em dash not
supposed to be capitalized, unless a specific
rule applies, typically when what follows is a proper noun.

Fix a few misuses of parentheses in following text.

(From yocto-docs rev: 5918f019f63f6e820b1168f4cc001faa1d1cdc6f)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/devtool-reference.rst

@@ -164,7 +164,7 @@ Adding a New Recipe to the Workspace Layer
 ==========================================
 
 Use the ``devtool add`` command to add a new recipe to the workspace
-layer. The recipe you add should not exist - ``devtool`` creates it for
+layer. The recipe you add should not exist --- ``devtool`` creates it for
 you. The source files the recipe uses should exist in an external area.
 
 The following example creates and adds a new recipe named ``jackson`` to