add a comment within .p10k.zsh explaining the use of "_joined" (#2332)

config/p10k-classic.zsh

@@ -168,6 +168,9 @@
   typeset -g POWERLEVEL9K_LEFT_SEGMENT_SEPARATOR='\uE0B0'
   # Separator between different-color segments on the right.
   typeset -g POWERLEVEL9K_RIGHT_SEGMENT_SEPARATOR='\uE0B2'
+  # To remove a separator between two segments, add "_joined" to the second segment name.
+  # For example: POWERLEVEL9K_RIGHT_PROMPT_ELEMENTS=(os_icon context_joined)
+
   # The right end of left prompt.
   typeset -g POWERLEVEL9K_LEFT_PROMPT_LAST_SEGMENT_END_SYMBOL='\uE0B0'
   # The left end of right prompt.

config/p10k-rainbow.zsh

@@ -165,6 +165,9 @@
   typeset -g POWERLEVEL9K_LEFT_SEGMENT_SEPARATOR='\uE0B0'
   # Separator between different-color segments on the right.
   typeset -g POWERLEVEL9K_RIGHT_SEGMENT_SEPARATOR='\uE0B2'
+  # To remove a separator between two segments, add "_joined" to the second segment name.
+  # For example: POWERLEVEL9K_RIGHT_PROMPT_ELEMENTS=(os_icon context_joined)
+
   # The right end of left prompt.
   typeset -g POWERLEVEL9K_LEFT_PROMPT_LAST_SEGMENT_END_SYMBOL='\uE0B0'
   # The left end of right prompt.