document wildcard usage in filename prefix

[mimir-d]

• I was looking to match more than a static prefix (with --prefix), like a filename regex.
• going through the code, gets me to Directory.GetFiles(options.Path!, searchPattern, fileEnumerationOptions)
• the dotnet docs for searchPattern say The search string to match against the names of subdirectories in path. This parameter can contain a combination of valid literal and wildcard characters, but it doesn't support regular expressions.
• it looks like the prefix and suffix strings are passed as is, so they actually accept wildcards (but no regex, which is sufficient)

Added help string to specify this.