Markdown can already add emphasis to formatted code blocks. It's not obvious or entirely trivial, but the basic method involves putting the code block (without the usual extra leading indent, and with all
> properly HTML-entity-encoded) inside nested
<code> tags, then using HTML emphasis, usually both
<i> together. It looks something like this:
You should change the values (see emphasis)
As you can see, this allows syntax highlighting to work as well. The Markdown source for that looks like this:
I would also recommend that you follow Tensibai's advice to make your descriptions more, well, descriptive. Formatting is a convenient shortcut, but it's not a perfect substitute for simple instruction. Adding extra UI features for a rarely-used and dubiously-helpful technique is not a good strategy in general. Everything in the UI needs to pull its own weight and be very common.
Finally, there are tools you can use to make inserting arrows easier for yourself. Consider ClipX + its Stickies plugin to allow simple Ctrl + Shift + V, A pasting of an arrow character, or modify any of several userscripts that adds toolbar buttons to insert an arrow at the cursor instead of whatever the script originally did.