Add more guidance to info field for search in Shiny app

[milanwiedemann]

@HelenCEBM noticed that searching fails when entering special characters such as {, (, or [.
To "fix", escape these with a backslash (e.g. \(). Characters with special meaning (e.g., ? in regex must also be escaped to be searched literally.

Search for context, this needs no changing:

opencodecounts/R/server.R

Lines 201 to 204 in dcf2de2

	if (!is.null(input$code_pattern_search) && input$code_pattern_search != "") {
	data <- data |>
	filter(grepl(paste("^", input$code_pattern_search, sep = ""), code, ignore.case = TRUE))
	}

Add more guidance for users here:

opencodecounts/R/ui.R

Line 67 in dcf2de2

"Enter search term(s). Multiple terms can be combined by using '|'.",