Documentation: Improve help output for examinemem (go-delve#2623)

Before this change when you typed `help` at the Delve prompt you would
only see the following:

```
examinemem (alias: x)  Examine memory:
```

Now with this patch the output is more descriptive:

```
examinemem (alias: x)  Examine raw memory at the given address.
```

Documentation/cli/README.md

@@ -43,7 +43,7 @@ Command | Description
 --------|------------
 [args](#args) | Print function arguments.
 [display](#display) | Print value of an expression every time the program stops.
-[examinemem](#examinemem) | Examine memory:
+[examinemem](#examinemem) | Examine raw memory at the given address.
 [locals](#locals) | Print local variables.
 [print](#print) | Evaluate an expression.
 [regs](#regs) | Print contents of CPU registers.
@@ -291,6 +291,8 @@ If locspec is omitted edit will open the current source file in the editor, othe
 Aliases: ed
 
 ## examinemem
+Examine raw memory at the given address.
+
 Examine memory:
 
 	examinemem [-fmt <format>] [-count|-len <count>] [-size <size>] <address>

pkg/terminal/command.go

@@ -487,7 +487,9 @@ Defines <alias> as an alias to <command> or removes an alias.`},
 If locspec is omitted edit will open the current source file in the editor, otherwise it will open the specified location.`},
 		{aliases: []string{"libraries"}, cmdFn: libraries, helpMsg: `List loaded dynamic libraries`},
 
-		{aliases: []string{"examinemem", "x"}, group: dataCmds, cmdFn: examineMemoryCmd, helpMsg: `Examine memory:
+		{aliases: []string{"examinemem", "x"}, group: dataCmds, cmdFn: examineMemoryCmd, helpMsg: `Examine raw memory at the given address.
+
+Examine memory:
 
 	examinemem [-fmt <format>] [-count|-len <count>] [-size <size>] <address>
 	examinemem [-fmt <format>] [-count|-len <count>] [-size <size>] -x <expression>