Allow MemorySwapMax be specified as a percentage

A configuration of:

```config
[Service]
MemoryMax=80%
MemorySwapMax=20%
```

seems to work:

```
systemctl status autofs.service
Memory: 134.6M (max: 2.6G swap max: 336.2M available: 2.4G)
```

The systemd documentation was lacking this information so I've corrected
that also.

* https://www.freedesktop.org/software/systemd/man/latest/systemd.resource-control.html#MemorySwapMax=bytes
* systemd/systemd#33593

REFERENCE.md

@@ -2888,7 +2888,7 @@ Struct[{
     Optional['MemoryHigh']                => Systemd::Unit::AmountOrPercent,
     Optional['MemoryMax']                 => Systemd::Unit::AmountOrPercent,
     Optional['MemoryLimit']               => Systemd::Unit::Amount,
-    Optional['MemorySwapMax']             => Systemd::Unit::Amount,
+    Optional['MemorySwapMax']             => Systemd::Unit::AmountOrPercent,
     Optional['TasksAccounting']           => Boolean,
     Optional['TasksMax']                  => Systemd::Unit::AmountOrPercent,
     Optional['IOAccounting']              => Boolean,

spec/type_aliases/systemd_unit_service_spec.rb

@@ -103,6 +103,8 @@
     )
   }
 
+  it { is_expected.to allow_value({ 'MemorySwapMax' => '80%' }) }
+
   it { is_expected.not_to allow_value({ 'CPUQuota' => 50 }) }
   it { is_expected.not_to allow_value({ 'CPUQuota' => '0%' }) }
   it { is_expected.not_to allow_value({ 'MemoryHigh' => '1Y' }) }

types/unit/service.pp

@@ -57,7 +57,7 @@
     Optional['MemoryHigh']                => Systemd::Unit::AmountOrPercent,
     Optional['MemoryMax']                 => Systemd::Unit::AmountOrPercent,
     Optional['MemoryLimit']               => Systemd::Unit::Amount,
-    Optional['MemorySwapMax']             => Systemd::Unit::Amount,
+    Optional['MemorySwapMax']             => Systemd::Unit::AmountOrPercent,
     Optional['TasksAccounting']           => Boolean,
     Optional['TasksMax']                  => Systemd::Unit::AmountOrPercent,
     Optional['IOAccounting']              => Boolean,