clarify comments for POWERLEVEL9K_DIR_MAX_LENGTH

Make it explicit that POWERLEVEL9K_DIR_MAX_LENGTH isn't
the only constraint that may cause directory truncation.

See the original description of #250 before this issue
was repurposed for something unrelated.

config/p10k-classic.zsh

@@ -252,7 +252,8 @@
   typeset -g POWERLEVEL9K_SHORTEN_DIR_LENGTH=1
   # Shorten directory if it's longer than this even if there is space for it. The value can
   # be either absolute (e.g., '80') or a percentage of terminal width (e.g, '50%'). If empty,
-  # directory will be shortened only when prompt doesn't fit.
+  # directory will be shortened only when prompt doesn't fit or when other parameters demand it
+  # (see POWERLEVEL9K_DIR_MIN_COMMAND_COLUMNS and POWERLEVEL9K_DIR_MIN_COMMAND_COLUMNS_PCT below).
   typeset -g POWERLEVEL9K_DIR_MAX_LENGTH=80
   # When `dir` segment is on the last prompt line, try to shorten it enough to leave at least this
   # many columns for typing commands.

config/p10k-lean.zsh

@@ -232,7 +232,8 @@
   typeset -g POWERLEVEL9K_SHORTEN_DIR_LENGTH=1
   # Shorten directory if it's longer than this even if there is space for it. The value can
   # be either absolute (e.g., '80') or a percentage of terminal width (e.g, '50%'). If empty,
-  # directory will be shortened only when prompt doesn't fit.
+  # directory will be shortened only when prompt doesn't fit or when other parameters demand it
+  # (see POWERLEVEL9K_DIR_MIN_COMMAND_COLUMNS and POWERLEVEL9K_DIR_MIN_COMMAND_COLUMNS_PCT below).
   typeset -g POWERLEVEL9K_DIR_MAX_LENGTH=80
   # When `dir` segment is on the last prompt line, try to shorten it enough to leave at least this
   # many columns for typing commands.