Properly tag monospaced text in man pages. (Improves HTML rendering) (#3509)

This would be immediately useful to improve the rendering of the man
pages in third-party websites such as manned.org and the arch man pagesk
- https://mankier.com/1/task
- https://man.archlinux.org/man/task.1

The regular console output, or websites which render the manpage in
monospaced fonts, will not be affected by this change.

This change could also help with eventually rendering the manpages in
the documentation website, and having a mix of monospaced and variable
width fonts.

To test the HTML output:
```bash
git clone git@github.com:jacksonp/manner.git # used by ManKier
./manner/manner.php task.1.in >| task.1.html && $BROWSER task.1.html
```

Co-authored-by: Sebastian Carlos <sebastiancarlos@gmail.com>

doc/man/task-sync.5.in

@@ -35,7 +35,9 @@ This is the default, and is appropriate for local synchronization.
 
 For synchronization to a server, a better solution is to run
 
+.nf
     $ task sync
+.fi
 
 periodically, such as via
 .BR cron (8) .
@@ -52,7 +54,9 @@ For most of these, you will need an encryption secret used to encrypt and
 decrypt your tasks. This can be any secret string, and must match for all
 replicas sharing tasks.
 
+.nf
     $ task config sync.encryption_secret <encryption_secret>
+.fi
 
 Tools such as
 .BR pwgen (1)
@@ -70,8 +74,10 @@ information from the server administrator:
 
 Configure Taskwarrior with these details:
 
+.nf
     $ task config sync.server.origin            <origin>
     $ task config sync.server.client_id         <client_id>
+.fi
 
 Note that the origin must include the scheme, such as 'http://' or 'https://'.
 
@@ -83,12 +89,16 @@ the bucket are adequate.
 
 Authenticate to the project with:
 
+.nf
     $ gcloud config set project $PROJECT_NAME
     $ gcloud auth application-default login
+.fi
 
 Then configure Taskwarrior with:
 
+.nf
     $ task config sync.gcp.bucket               <bucket-name>
+.fi
 
 However you can bring your own service account credentials if your 
 `application-default` is already being used by some other application
@@ -124,8 +134,10 @@ Select the following permissions:
 
 Then configure Taskwarrior with:
 
+.nf
     $ task config sync.gcp.bucket               <bucket-name>
     $ task config sync.gcp.credential_path       <absolute-path-to-downloaded-credentials>
+.fi
 
 .SS Local Synchronization
 
@@ -133,7 +145,9 @@ In order to take advantage of synchronization's side effect of saving disk
 space without setting up a remote server, it is possible to sync tasks locally.
 To configure local sync:
 
+.nf
     $ task config sync.local.server_dir /path/to/sync
+.fi
 
 The default configuration is to sync to a database in the task directory
 ("data.location").
@@ -162,11 +176,15 @@ invent their own "encryption_secret".
 If you run multiple clients that sync to the same server, you will need to run
 this command on your primary client (the one you use most often):
 
+.nf
     $ task config recurrence on
+.fi
 
 And on the other clients, run:
 
+.nf
     $ task config recurrence off
+.fi
 
 This protects you against the effects of a sync/duplication bug.
 
@@ -185,7 +203,9 @@ modifying the same task on two machines, without an intervening sync.
 Setup simply involves creating the directory and modifying your data.location
 configuration variable like this:
 
+.nf
     $ task config data.location /path/to/shared/directory
+.fi
 
 Strengths:
 .br