New review comment by gt7-void on void-packages repository

https://github.com/void-linux/void-packages/pull/23108#discussion_r475272167

Comment:
I just took the chance to make some (mostly) noop changes to the template to make xlint happy. The proposed commit fixes all of the following warnings in the current template:
```
$ xlint srcpkgs/mdocml/template
srcpkgs/mdocml/template: Place checkdepends= after depends=
srcpkgs/mdocml/template: Place provides= after checksum=
srcpkgs/mdocml/template:14: short_desc should not start with an article
srcpkgs/mdocml/template:33: trailing whitespace
srcpkgs/mdocml/template:40: custom variables should use _ prefix: PREFIX=/usr
srcpkgs/mdocml/template:41: custom variables should use _ prefix: SBINDIR=/usr/bin
srcpkgs/mdocml/template:42: custom variables should use _ prefix: MANDIR=/usr/share/man
srcpkgs/mdocml/template:43: custom variables should use _ prefix: OSNAME="Void Linux"
srcpkgs/mdocml/template:44: custom variables should use _ prefix: BINM_SOELIM=mandoc-soelim
srcpkgs/mdocml/template:45: custom variables should use _ prefix: MANM_ROFF="mandoc_roff"
srcpkgs/mdocml/template:49: custom variables should use _ prefix: HAVE_REWB_BSD=0
srcpkgs/mdocml/template:54: trailing whitespace
```

What's the recommended way to fix the warnings for lines 40-49?
The `-` means to strip leading tab characters (https://www.gnu.org/software/bash/manual/html_node/Redirections.html#Here-Documents)

Several templates use this pattern of indented heredoc (for one example see the git template).

I think the indented version is easier to read (either with two tabs or just one tab as in the git template) but I don't really care either way.