Docs: improve cross-platform help messaging

tldr; don't use npm for multi-platform installs

lib/sharp.js

@@ -49,11 +49,11 @@ if (sharp) {
     help.push(
       '- Ensure optional dependencies can be installed:',
       '    npm install --include=optional sharp',
-      '  or',
       '    yarn add sharp --ignore-engines',
+      '- Ensure your package manager supports multi-platform installation:',
+      '    See https://sharp.pixelplumbing.com/install#cross-platform',
       '- Add platform-specific dependencies:',
       `    npm install --os=${os} --cpu=${cpu} sharp`,
-      '  or',
       `    npm install --force @img/sharp-${runtimePlatform}`
     );
   } else {
@@ -61,7 +61,6 @@ if (sharp) {
       `- Manually install libvips >= ${minimumLibvipsVersion}`,
       '- Add experimental WebAssembly-based dependencies:',
       '    npm install --cpu=wasm32 sharp',
-      '  or',
       '    npm install --force @img/sharp-wasm32'
     );
   }
@@ -103,7 +102,7 @@ if (sharp) {
   }
   help.push(
     '- Consult the installation documentation:',
-    '    https://sharp.pixelplumbing.com/install'
+    '    See https://sharp.pixelplumbing.com/install'
   );
   throw new Error(help.join('\n'));
 }