change the default of history.isolation (#9762)

# Description

This PR just fixes the default value of history.isolation and adds a few
more comments. isolation isn't available in plaintext so it should be
defaulted to off/false.

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crates/nu-utils/src/sample_config/default_config.nu

@@ -249,7 +249,7 @@ $env.config = {
         max_size: 100_000 # Session has to be reloaded for this to take effect
         sync_on_enter: true # Enable to share history between multiple sessions, else you have to close the session to write history to file
         file_format: "plaintext" # "sqlite" or "plaintext"
-        isolation: true # true enables history isolation, false disables it. true will allow the history to be isolated to the current session. false will allow the history to be shared across all sessions.
+        isolation: false # only available with sqlite file_format. true enables history isolation, false disables it. true will allow the history to be isolated to the current session using up/down arrows. false will allow the history to be shared across all sessions.
     }
 
     completions: {