Add clarity to docs surrounding command line arguments, suggest data-dir when syncing, and polishing up for mainnet (#2527)

* mainnet

* add clarity to command line rules

* add datadir suggestion when syncing

* remind to pass with all calls

* noCommand added into troubleshooting

* review

Co-authored-by: 0xmiel <sacha.saint-leger@ethereum.org>

* review

Co-authored-by: 0xmiel <sacha.saint-leger@ethereum.org>

* review

Co-authored-by: 0xmiel <sacha.saint-leger@ethereum.org>

* review

Co-authored-by: 0xmiel <sacha.saint-leger@ethereum.org>

* typo

* /lib/systemd/system/ lets you enable on startup

Co-authored-by: 0xmiel <sacha.saint-leger@ethereum.org>
Co-authored-by: Jacek Sieka <arnetheduck@gmail.com>
Co-authored-by: 0xmiel <sacha@status.im>

docs/the_nimbus_book/src/beacon-node-systemd.md

@@ -10,7 +10,7 @@ Systemd is used in order to have a command or program run when your device boots
 
 > ⚠️  If you wish to run the service with metrics enabled, you'll need to replace `--metrics:off` with `--metrics:on` in the service file below. See [here](./metrics-pretty-pictures.md) for more on metrics.
 
-Create a `systemd` service unit file -- `nimbus-eth2-pyrmont.service` -- and save it in `/etc/systemd/system/`.
+Create a `systemd` service unit file -- `nimbus-eth2-pyrmont.service` -- and save it in `/lib/systemd/system/`.
 
 The contents of the file should look like this:
 
@@ -45,6 +45,7 @@ Where you should replace:
 
 > **N.B.** If you're running Nimbus on a Pi, your `<BASE-DIRECTORY>` is `/home/pi/nimbus-eth2/` and your `<USERNAME>` is `pi`
 
+> If you want to run on mainnet, simply replace all instances of `pyrmont` with `mainnet`.
 
 ### 2. Notify systemd of the newly added service
 

docs/the_nimbus_book/src/options.md

@@ -1,5 +1,7 @@
 # Command line options
 
+> ⚠️  If you want to add your own options, remember that the format is `--foo=bar`, `--baz`, or `--foo-bar=qux`
+
 You can pass any `nimbus_beacon_node` options to the `pyrmont` and `mainnet` scripts. For example, if you wanted to launch Nimbus on mainnet with different base ports than the default `9000/udp` and `9000/tcp`, say `9100/udp` and `9100/tcp`, you would run:
 
 ```

docs/the_nimbus_book/src/start-syncing.md

@@ -42,7 +42,7 @@ INF 2020-12-01 11:26:31.000+00:00 Slot start                                 top
 INF 2020-12-01 11:26:36.285+00:00 Slot end                                   topics="beacnde" tid=21815 file=nimbus_beacon_node.nim:593 slot=96567 nextSlot=96568 head=b54486c4:96563 headEpoch=3017 finalizedHead=2f5d12e4:96479 finalizedEpoch=3014
 ...
 ```
-
+> If you want to put the database somewhere else, (e.g. an external ssd) pass the `--data-dir=/your/path`. ⚠️ If you do this, remember to pass this flag to **all** your nimbus calls.
 
 ### Command line options
 

docs/the_nimbus_book/src/troubleshooting.md

@@ -50,6 +50,11 @@ If you're experiencing a low peer count, you may be behind a firewall. Try resta
 ```
 ./run-pyrmont-beacon-node.sh --nat:extip:35.124.65.104
 ```
+### noCommand does not accept arguments
+
+If, on start,  you see
+```The command 'noCommand' does not accept arguments```
+Double check to see if your command line flags are in the correct format, i.e. `--foo=bar`, `--baz`, or `--foo-bar=qux`.
 
 ### Address already in use error
 
@@ -91,6 +96,7 @@ The first line in the output will give you internet time. And the second line wi
 
 ### Eth1 chain monitor failure
 
+
 If you see an error that looks like the following:
 
 ```
@@ -105,5 +111,3 @@ It's because your node can't connect to the web3 provider you have specified. Pl
 
 We have seen reports of extremely degraded performance when using several types of USB3.0 to SSD adapter or when using native USB3.0 disk drives. [This post](https://www.raspberrypi.org/forums/viewtopic.php?t=245931#p1501426) details why there is a difference in behaviour from models prior to Pi 4 and the recommended workaround.
 
-
-