doc: Always use @t in @pygments environments for non-PDF output

A [forthcoming change in Pygments]
(pygments/pygments#2283) improves rendering of `--`,
if representing an attached tenuto, as bold using LilyPond's `@tb` macro
instead of `@t`.

Previously, the `@tb` (and `@tbsl`) macro did nothing for non-PDF output,
simply passing through its argument.  However, within the `@pygments`
environment, which in turn is based on the `@format` environment, `makeinfo`
converts blank `--` to en-dashes (U+2013), which we don't want for LilyPond
code listings.

Documentation/en/common-macros.itexi

@@ -748,7 +748,7 @@ UNTRANSLATED NODE: IGNORE ME
 
 @ifnottex
 @macro tb {arg}
-\arg\
+@t{\arg\}
 @end macro
 @end ifnottex
 
@@ -763,7 +763,7 @@ UNTRANSLATED NODE: IGNORE ME
 
 @ifnottex
 @macro tbsl {arg}
-\arg\
+@t{\arg\}
 @end macro
 @end ifnottex