cmd/shfmt: document when --filename is needed for EditorConfig support

EditorConfig files are found based on a script's absolute path,
and the EditorConfig patterns are usually filename-based as well.

When formatting standard input, there is no known filename or path.
Sometimes there's a need to format a stream of bytes in memory
without having to place them on a file, which is why we support stdin.

We added the --filename flag for this purpose a long time ago,
and the tests already verified this fact - we just hadn't documented it.

While here, also add a test case for absolute paths.

Fixes #1055.

cmd/shfmt/shfmt.1.scd

@@ -73,6 +73,9 @@ predictable. Some aspects of the format can be configured via printer flags.
 *-filename* str
 	Provide a name for the standard input file.
 
+	Use of this flag is necessary for EditorConfig support to work,
+	since EditorConfig files are found relative to the location of a script.
+
 ## Printer flags
 
 *-i*, *--indent* <uint>

cmd/shfmt/testdata/script/editorconfig.txtar

@@ -14,6 +14,10 @@ stdin stdin-filename-bash
 ! exec shfmt -filename=foo_posix.sh
 stderr '^foo_posix.sh:.* arrays are a bash'
 
+stdin stdin-filename-bash
+! exec shfmt -filename=${WORK}/foo_posix.sh
+stderr ^${WORK@R}/'foo_posix.sh:.* arrays are a bash'
+
 # Using a file path should use EditorConfig, including with the use of flags
 # like -l.
 exec shfmt input.sh